Add better support for Cloud Pub/Sub

[kamalaboulhosn]

Is your feature request related to a problem? Please describe.
Cloud Pub/Sub has a more advanced hand-written client library that handles operations on both the publish and subscribe side. For the admin operations, the Google API tool is sufficient. However, for publishing and pulling, it has some deficiencies including:

1. Problems with publish where the topic cannot be set because it is topic in the API definition instead of topicId.
2. The publish data must be base64 encoded when using the Google API tool directly.
3. Pulling and acking are related operations and one should be able to ack messages automatically (as is available in gcloud).

Describe the solution you'd like
It would be nice to have a custom tool for Cloud Pub/Sub that handles the above.

Describe alternatives you've considered
For 1. and 2., considered making changes to the Google API tool, but the fixes are somewhat specific to Pub/Sub. The third item could not be dealt with by changes to the existing tool.