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

**Is your feature request related to a problem? Please describe.**
[new-client.py](https://github.com/googleapis/google-cloud-java/blob/main/generation/new_client/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.