Integrate into protoc-gen-prost-validate

Hi! I saw your comment today [on the protoc](https://github.com/bufbuild/protovalidate/issues/33#issuecomment-2403643276)

This issue asks to integrate with https://github.com/neoeinstein/protoc-gen-prost/tree/main/protoc-gen-prost-validate

I believe first some work is required to support prost generated files instead of straight up file descriptor sets.

Note that ideally this protobuf plugin should be usable through `protoc` or `buf` and generate files in `OUT_DIR`, as is the case with the other protoc plugins in @neoeinstein's repo.

I'd like to help! :)

Start by reading the protoc-gen-prost-validate integration in the linked neoeinstein/protoc-gen-prost repository and determine how it currently handles file descriptor sets versus prost-generated files. Done means the plugin works through protoc or buf and generates the expected files in OUT_DIR, matching the behavior of the other protoc plugins in that repository.

Written by the indexing model from the issue text.