http2: refactor and cleanup http2

* cleanup constants in http2 binding
  The error constants were just doing some weird things. Cleanup
  and improve maintainability.
* simplify settings to reduce duplicate code
* improve style consistency and correctness
  Use snake_case for getters and setters at c++ level,
  avoid unnecessary use of enums,
  use consistent style for exported vs. internal constants,
  avoid unnecessary memory info reporting,
  use setters/getters for flags_ for improved code readability
* make EmitStatistics function private
* un-nest Http2Settings and Http2Ping
* refactoring and cleanup of Http2Settings and Http2Ping
* avoid ** syntax for readability
  The **session and **stream syntax for getting the underlying
  nghttp2 pointers is not ideal for readability
* use const references for Http2Priority
* remove unnecessary GetStream function
* refactor Http2Scope to use BaseObjectPtr
* move utility function to anonymous namespace
* refactor and simplify Origins
* Use an AllocatedBuffer instead of MaybeStackBuffer
* Use a const reference instead of pointer
* use BaseObjectPtr for Http2Streams map
* move MemoryInfo impl to cc

Signed-off-by: James M Snell <jasnell@gmail.com>

PR-URL: #32884
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>