sdk-platform-java: evaluate how much work it would be to deprecate *_gapic.yaml files

Rather than accepting an *_gapic.yaml file as input, how much work would it be to pass each of the values as arguments?

This would allow us to store these files in librarian.yaml, rather than having to reconstruct the *_gapic.yaml file from scratch when we migrate Java to Librarian.

Start by tracing the current *_gapic.yaml input handling and the librarian.yaml migration path mentioned in the issue. Document the required arguments and migration impact; done means an agreed estimate and scope for deprecating the files.

Written by the indexing model from the issue text.