Merge pull request grpc#15374 from mehrdada/v1.12.x

Bump v1.12.x to v1.12.0

Merge pull request grpc#15371 from mehrdada/v1.11.x

Backport 15362 to 1.11.x

Merge pull request grpc#15263 from muxi/backport-15262

Backport 15262: Provide protocol for initializer of generated messages

Merge pull request grpc#15257 from mehrdada/v1.11.x

Bump v1.11.x to v1.11.1-pre1

Merge pull request grpc#15035 from mehrdada/v1.11.x

Bump v1.11.x to 1.11.0   #gorgeous

Merge pull request grpc#14997 from mehrdada/v1.11.x

 Bump v1.11.x to v1.11.0-pre2

Merge pull request grpc#14946 from apolcyn/backport_14922

Backport grpc#14922 to 1.11.x

Merge pull request grpc#14944 from mehrdada/v1.10.x

Bump v1.10.x to v1.10.1

Merge pull request grpc#14833 from mehrdada/v1.10.x

 Bump v1.10.x to 1.10.1-pre1

Merge pull request grpc#14545 from mehrdada/final-1-10-0

Bump version to 1.10.x   #glamorous