Enable type-safe interoperability between different independent Python/C++ bindings systems. (#5296)

* `self.__cpp_transporter__()` proof of concept: Enable passing C++ pointers across extensions even if the `PYBIND11_INTERNALS_VERSION`s do not match.

* Include cleanup (mainly to resolve PyPy build failures).

* Fix clang-tidy errors.

* Resolve `error: extra

* factor out platform_abi_id.h from internals.h (no functional changes)

* factor out internals_version.h from internals.h (no functional changes)

* Update CMakeLists.txt, tests/extra_python_package/test_files.py

* Revert "factor out internals_version.h from internals.h (no functional changes)"

This reverts commit 3ccea8cd4302cfaf2b6842f77ee3a2eefd61182a.

* Remove internals_version.h from CMakeLists.txt, tests/extra_python_package/test_files.py

* `.__cpp_transporter__()` implementation: compare `pybind11_platform_abi_id`, `cpp_typeid_name`

* Add PremiumTraveler

* Rename test_cpp_transporter_traveler_type.h -> test_cpp_transporter_traveler_types.h

* Expand tests: `PremiumTraveler`, `get_points()`

* Shuffle order of tests (no real changes).

* Move `__cpp_transporter__` lambda to `py::cpp_transporter()` regular function.

* Use `type_caster_generic::load(self)` instead of `cast<T *>(self)`

* Pass `const std::type_info *` via `py::capsule` (instead of `cpp_typeid_name`).

* Make platform_abi_id.h completely stand-alone.

* rename exo_planet.cpp -> exo_planet_pybind11.cpp

* Add exo_planet_c_api.cpp (incomplete).

* Fix silly oversight (wrong filename in `#include`).

* Resolve clang-tidy errors:

```
/__w/pybind11/pybind11/tests/exo_planet_c_api.cpp:10:18: error: 'wrapGetLuggage' is a static definition in anonymous namespace; static is redundant here [readability-static-definition-in-anonymous-namespace,-warnings-as-errors]
   10 | static PyObject *wrapGetLuggage(PyObject *, PyObject *) { return PyUnicode_FromString("TODO"); }
      | ~~~~~~           ^
/__w/pybind11/pybind11/tests/exo_planet_c_api.cpp:14:20: error: 'ThisMethodDef' is a static definition in anonymous namespace; static is redundant here [readability-static-definition-in-anonymous-namespace,-warnings-as-errors]
   14 | static PyMethodDef ThisMethodDef[]
      | ~~~~~~             ^
/__w/pybind11/pybind11/tests/exo_planet_c_api.cpp:17:27: error: 'ThisModuleDef' is a static definition in anonymous namespace; static is redundant here [readability-static-definition-in-anonymous-namespace,-warnings-as-errors]
   17 | static struct PyModuleDef ThisModuleDef = {
      | ~~~~~~                    ^
```

* Implement exo_planet_c_api GetLuggage(), GetPoints()

* Move new code from test_cpp_transporter_traveler_bindings.h to pybind11/detail/type_caster_base.h, under the name `class_dunder_cpp_transporter()`

* Fix oversight.

* Unconditionally add `__cpp_transporter__` method to all `py::class_` objects, but do not include that magic method in docstring signatures.

* Back out pybind11/detail/platform_abi_id.h for now. Maximizing reusability can be handled separately, later.

* Small cleanup.

* Restore and add to `test_call_cpp_transporter_*()`

* Ensure https://github.com/pybind/pybind11/issues/3788 does not bite again.

* `class_dunder_cpp_transporter()`: replace `obj.cast<std::string>()` with `std::string(obj)`

* Add (simple) copyright notices in all newly added files.

* Globally replace cpp_transporter with cpp_conduit

* style: pre-commit fixes

* IWYU fixes

* Rename `class_dunder_cpp_conduit()` -> `cpp_conduit_method()`

* Change `pybind11_platform_abi_id`, `pointer_kind` argument types from `str` to `bytes`.

This avoids the unicode decode/encode roundtrips:

* More robust (no decode/encode errors).

* Minor runtime optimization.

* Systematically rename `cap_cpp_type_info` -> `cpp_type_info_capsule` (no functional changes).

* Systematically replace `cpp_type_info_capsule` `name`: `"const std::type_info *"` -> `typeid(std::type_info).name()` (this IS a functional change).

This provides an extra layer of protection against C++ ABI mismatches:

* The first and most important layer is that the `PYBIND11_PLATFORM_ABI_ID`s must match between extensions.

* The second layer is that the `typeid(std::type_info).name()`s must match between extensions.

* Fix sort order accident in tests/CMakeLists.txt

* Apply suggestions from code review

Co-authored-by: Aaron Gokaslan <aaronGokaslan@gmail.com>

* style: pre-commit fixes

* refactor: rename to _pybind_conduit_v1_

Signed-off-by: Henry Schreiner <henryschreineriii@gmail.com>

* Add test_home_planet_wrap_very_lonely_traveler(), test_exo_planet_pybind11_wrap_very_lonely_traveler()

* Resolve clang-tidy errors:

```
/__w/pybind11/pybind11/tests/test_cpp_conduit_traveler_bindings.h:39:32: error: parameter 'm' is passed by value and only copied once; consider moving it to avoid unnecessary copies [performance-unnecessary-value-param,-warnings-as-errors]
   10 |     py::class_<LonelyTraveler>(m, "LonelyTraveler");
      |                                ^
      |                                std::move( )
/__w/pybind11/pybind11/tests/test_cpp_conduit_traveler_bindings.h:43:52: error: parameter 'm' is passed by value and only copied once; consider moving it to avoid unnecessary copies [performance-unnecessary-value-param,-warnings-as-errors]
   43 |     py::class_<VeryLonelyTraveler, LonelyTraveler>(m, "VeryLonelyTraveler");
      |                                                    ^
      |                                                    std::move( )
```

---------

Signed-off-by: Henry Schreiner <henryschreineriii@gmail.com>
Co-authored-by: Ralf W. Grosse-Kunstleve <rwgk@google.com>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Henry Schreiner <HenrySchreinerIII@gmail.com>
Co-authored-by: Aaron Gokaslan <aaronGokaslan@gmail.com>