alts: add ComputeEngineChannelBuilder

[jiangtaoli2016]

Add ComputeEngineChannelBuilder. This is similar to GoogleDefaultChannelBuilder, except that ComputeEngineChannelBuilder only runs on GCE and only fetches call credentials from metadata server.

[jiangtaoli2016]

Hold on review. Let me fix presubmit error first.

[ejona86]

@jiangtaoli2016, those don't look like failures with your code. Try to rebuild them. Although the linux_artifacts looks broken for an unrelated reason; rebuilding won't fix it (but you can ignore it for now).

[jiangtaoli2016]

Thanks @ejona86 for quick review.
@apolcyn Could you start using this new channel builder in your staging test?

[apolcyn]

Thanks for the quick turnaround on this. I'll update tests to start using this

[apolcyn]

Actually for tests, can we follow up this PR to add support for using this new channel builder in the interop client? (probably around https://github.com/grpc/grpc-java/blob/master/interop-testing/src/main/java/io/grpc/testing/integration/TestServiceClient.java#L372)

[jiangtaoli2016]

I forgot about that. Let me update the interop client with a new customCredentialTypes = "compute_engine_credentials" on Java side. @apolcyn can you update the python script with this new type after I submit java PR?

[apolcyn]

yeah, I can update tests once that's in