Reduce customizations for generated clients `grafeas` and `containeranalysis`

[parthea]

There are currently a lot of customizations for the grafeas client which were introduced in https://github.com/googleapis/google-cloud-python/pull/8186/ via the synth.py file. This issue tracks adding support for the customizations in gapic-generator-python rather than using client-specific post processing.

There are similar customizations in google-cloud-containeranalysis related to grafeas https://github.com/googleapis/python-containeranalysis/blob/023761bba8644aa75ea90a5c038ecc50a3a0bfad/owlbot.py#L49