Add support for `absl::InfiniteFuture`, `absl::InfinitePast`, matching established Google-internal conventions under absl/python.

Also replace floating-point arithmetic in the handling of microseconds with integer arithmetic, to sidestep subtle round-off issues. See the source code comment in the new `internal::GetTimestampMicrosFromDateTimeObj()` function for details.

PiperOrigin-RevId: 529169464