Update new-client.py to set transport based on Bazel

[alicejli]

Is your feature request related to a problem? Please describe.
new-client.py generates the repo-metadata.json file with the transport option set to grpc was default. This can lead to discrepancies between repo-metadata.json and the BUILD.bazel file which is technically the source of truth.

For instance, for java-api-gateway: the repo-metadata.json file shows transport: grpc: https://github.com/googleapis/java-api-gateway/blob/main/.repo-metadata.json#L8, however the BUILD.bazel file shows that actually transport: grpc+rest: https://github.com/googleapis/googleapis/blob/master/google/cloud/apigateway/v1/BUILD.bazel#L96.

This will not fix the issue where the BUILD.bazel file gets updated post initial library generation to a different transport; that can be addressed in a follow up.

Describe the solution you'd like
new-client.py should generate the transport option based on the BUILD.bazel file if possible.

Alternatively, if there's a way to automatically update repo-metadata.json based on the BUILD.bazel file in general, that could solve both new client generation and existing library updates.

[meltsufin]

This is a great observation! I think it points to the problem with the generator not doing the full job, and having to rely on post-processing to finish the generator's job. The generator should probably generate repo-metadata.json. Alternatively, new-client.py can try to auto-detect the transport based on the generated classes.

[suztomo]

Who suffer from the wrong value in the repo-metadata.json files?