Add grpc-testing-proto to grpc-all

This is to expose the generated code's javadoc and suppress the generated code's jacoco results. Currently javadoc is not generating documentation for the generated code.

I expect this to be non-trivial, but not especially difficult.

Start by locating the build definitions for grpc-testing-proto and grpc-all, then inspect how generated code is handled for Javadoc and JaCoCo. Confirm the module is exposed through grpc-all, generated-code Javadoc is produced, and its JaCoCo results are suppressed; run the relevant build checks.

Vom Indexierungsmodell aus dem Issue-Text verfasst.