feat: pass a CallCredentials to grpc-java for DirectPath

[mohanli-ml]

DirectPath is going to support non-default service account. In grpc-java, we allow GoogleDefaultChannelCredentials to take a CallCredentials as input. Thus, gax-java should pass a CallCredentials to grpc-java, and no longer use ComputeEngineChannelBuilder.

Relevant grpc-java PR: grpc/grpc-java#8548.

[chanseokoh]

@mohanli-ml grpc/grpc-java#8548 is merged. What's the status of this PR?

[mohanli-ml]

This PR is obsolete. Specifically, gax-java should use builder = GoogleDefaultChannelCredentials.newBuilder().callCredentias(MoreCallCredentials.from(credentials)).build(); instead of builder = ComputeEngineChannelBuilder.forAddress(serviceAddress, port);. I will update this PR.

Now I am waiting for the release of grpc/grpc-java#8548, and then gax-java to depend on the latest version of grpc-java. Once we have that I can update this PR and test should pass. Then this PR is ready for review.

[chanseokoh]

There's a direct conflict from the main branch. I think you should sort it out first.