feat(cloud): Wrapper clients provide better introspection to determine availability of client objects

[dazuma]

Currently the client object factory methods in wrapper clients just expect that the underlying GAPIC provides the correct classes, and will raise LoadErrors or NameErrors if used with an out-of-date GAPIC. In particular, the unit tests for wrappers will fail if the GAPIC is out of date. This causes problems when a new service is added to an API, and gets added to the GAPIC and wrapper simultaneously. The wrapper tests fail until the GAPIC is updated and released, and we also typically go through the extra step of updating the version requirement in the dependency. This is a recurring source of toil.

This PR improves the situation by:

• Providing additional documentation calling out that an out-of-date GAPIC could cause the wrapper's factory method to fail, and noting what should be done.
• Providing new methods in the wrapper to test whether a service is available in the given API version and transport type.
• Updating the wrapper unit tests to skip tests when a service is unavailable.

This PR also updates the wrapper README with a clarification that debug logging also requires a recent GAPIC.

[aandreassa]

Typo in sufficiently.

[dazuma]

Whoa, thanks for catching that!