Audit `additional_proto_files`

LROs have an annotation for their returned metadata type. Proto syntax does not require `import`s for the proto files that define these metadata types. In C++ we have to include these files.

Our intention for `additional_proto_files` was to add includes for these extra `*.pb.h` headers.

Turns out that when we feed them into `protoc` we treat them the same as any other service's proto file.

https://github.com/googleapis/google-cloud-cpp/blob/a86e887fe2756935a51d4d93b42e45a0fdde7812/generator/standalone_main.cc#L314-L317

This leads to some weirdness in the generator config. In `notebooks` and `run`, we rely on this behavior to generate some clients (and avoid #7997). In `containeranalysis`, we accidentally generate a client for `grafeas` which we did not intend to. Oops.

We should sort this out.

Read generator/standalone_main.cc around lines 314-317, then inspect the generator configuration for notebooks, run, and containeranalysis. Determine how additional_proto_files are passed to protoc versus used for extra headers. Done means preserving the intended generated clients while avoiding the unintended grafeas client.

Written by the indexing model from the issue text.