Squash generator decorators together to reduce number of TUs to compile

[alevenberg]

Googlers can see: go/cloud-cxx:reducing-build-times

Milestone 1 - GAPICs

• Move speech/v2/streaming.cc to internal
• Fix gRPC/REST name clash in metadata decorator
• Refactor GAPIC's BUILD.bazel files #14171
• Add new service code generator. Turned on only if: !generate_rest_transport && !omit_client. Fix build files as needed.

Milestone 2 - (DI)REGAPICs

• Turn on for REST. (sql, compute)

Milestone 3 - Veneers

Add handwritten conglomerate files and update build files. See: #14242

Cleanups

• (optional) Clean up handling of streaming.cc
• Emit NOLINT locally in conglomerate source files, instead of suppressing bugprone-suspicious-include globally
• Copyright years should be max(2024, year) for conglomerate source files