fix: add api_version to gapic_metadata.json

[noahdietz]

Populates the api_version field on a per-service basis to the generated gapic_metadata.json using the value from the service-level annotation google.api.api_version already parsed for use in the X-Goog-Api-Version/$apiVersion system parameter (as of #1999).

One unit test for gapic_metadata generation is updated to include an api_version proto service extension for populating. The other omits it which subsequently tests the negative case. None of the APIs used for integration test goldens currently set the annotation, so those are not updated.

It's presence in gapic_metadata.json will allow parsers to correlate service client to API version. It is not in use by any API yet.

Internal bug http://b/452365074.