Squash generator decorators together to reduce number of TUs to compile

[alevenberg]

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

Milestone 1 - GAPICs

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

Milestone 2 - (DI)REGAPICs

• [x] Turn on for REST. (sql, compute)

Milestone 3 - Veneers

Add handwritten conglomerate files and update build files for:

• [ ] bigtable
• [ ] pubsub
• [ ] pubsublite
• [ ] spanner
• [ ] storage (gRPC only)

Note that this milestone is not blocked on either M1 or M2.

Cleanups

• [ ] (optional) Clean up handling of streaming.cc
• [x] 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