transport/grpcframer: create grpcframer package

[printchard]

Create grpcframer package with type definitions.

Fixes: #7376 As part of the effort to reduce buffer copies in the transport layer of gRPC-Go, this PR declares the new API along with the necessary methods to implement a new HTTP/2 Framer.

RELEASE NOTES: none

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 10.00000% with 36 lines in your changes missing coverage. Please review.

Project coverage is 81.42%. Comparing base (bdd707e) to head (e450516). Report is 19 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #7397    +/-   ##
========================================
  Coverage   81.42%   81.42%            
========================================
  Files         348      352     +4     
  Lines       26744    26917   +173     
========================================
+ Hits        21775    21916   +141     
- Misses       3779     3810    +31     
- Partials     1190     1191     +1     

Files	Coverage Δ
internal/transport/grpchttp2/errors.go	100.00% <100.00%> (ø)
internal/transport/grpchttp2/framer.go	0.00% <0.00%> (ø)

... and 40 files with indirect coverage changes

[easwars]

And vet-proto is failing because we made a recent change to the codegen. So, it might be worth rebasing to master.

[easwars]

Also, i went ahead and marked some comments as resolved to reduce the noise in the review.

[easwars]

@zasweq : You seem to have requested changes on this PR. Do you mind taking a pass? Thanks.