codegen to allow hermetic build configs to setup optional googleapis version on library level

This is a followup to hermetic build features.
To allow for library level googleapis_committish to take precedent (optional config in generation_config.yaml).

context: https://github.com/GoogleCloudPlatform/spring-cloud-gcp/pull/2764#discussion_r1571196156

Start by reading the discussion linked from PR 2764 and inspect generation_config.yaml to understand the existing hermetic build configuration. The work is done when an optional library-level googleapis_committish setting is supported and takes precedence over the broader configuration.

Written by the indexing model from the issue text.