add changes to allow downstream support for grpc gateway

[ewhauser]

@pcj I pulled the functionality from the two gRPC Gateway PRs that is necessary for downstream consumers to be apply to run them in their own repository. Since those two PRs are quite large, I thought it might be easier to get this merged. This would allow us to fully get rid of our fork.

Thanks!

[pcj]

Hey, thanks for the work you've put into this! This does look like at least two PRs (ts-proto, direct_mode) that we'd want to get separated out.

For direct mode, I'm not certain what use case it's solving. Can you create a golden example? This would involve creating a new subdirectory in example/golden/testdata. The idea is that the BUILD.in files represent the state before running gazelle, and BUILD.out represents the state afterwards. The golden_test.go file will pick it up and run it to compare the expected state matched the actual state.

This is typically paired with a gazelle_testdata_example (see example/golden/BUILD.bazel). This test assembles the set of files from BUILD.out and actually runs bazel build on it to ensure that it actually builds.

Since the purpose of this PR is to make sure grpc-gateway works, we should have a full golden test and testdata example test of that as well, don't you think?

[pcj]

A bit old, but let's revisit the work needed to get this rebased and resolved.

[ewhauser]

I think this can be closed out. I have this working with the patch in the latest versions.