absl::CordBuffer holds data for eventual inclusion within an existing
absl::Cord. CordBuffers are useful for building large Cords that may
require custom allocation of its associated memory, a pattern that is
common in zero-copy APIs.
PiperOrigin-RevId: 453212229
Change-Id: I6a8adc3a8d206691cb1b0001a9161e5080dd1c5f