Taking over work started in googleforgames#1144, so we can upgrade Opencensus

- Updates gRPC in all places, which updates proto and grpc-gateway
- Regenerates the code for all gRPC SDKs as well
- Need to workaround a issue with the /watch Swagger codegen, as the swagger changed between versions.
  This likely means the JSON structure from grpc-gateway for streaming operations also changed.
  This is a breaking change, but we have no control over it (not sure if anyone is using this
  specific REST api endpoint, so it's probably a non issue).
- Use the C++ library requisites file in the cloud build cache key, so it gets updated
  as dependencies get updated.

Closes googleforgames#1214

Co-authored-by: Cyril Tovena <cyril.tovena@gmail.com>