Port buf to use protoplugin

[bufdev]

No description provided.

[mfridman]

nice to have this logic consolidated in one place.

did we need to address the TODO re. source-retention options in the protoc exec?

[bufdev]

Yes we do, caught up with @jhump offline and I have a path forward, will do some testing later

[bufdev]

Merging so we can test with this today, will follow up

[bufdev]

Verified with @jhump that source-retention options are propagated to protoc as we expect, we did manual QA (this would be incredibly hard to add an automated test for).