Reuse containers here for fulcio/rekor proto generation

[loosebazooka]

Proto builder containers here provide

• Access to google, googleapis proto defs
• An update mechanism

The go protobuf container generated here is almost sufficient for generating the rekor(v2)/fulcio code except it needs

• [ ] tool: github.com/grpc-ecosystem/grpc-gateway/v2/protoc-gen-grpc-gateway
• [ ] tool: github.com/grpc-ecosystem/grpc-gateway/v2/protoc-gen-openapiv2

publish image somewhere

• [ ] ghcr?
• [ ] AR in sigstore.dev GCP project

Alternatively, just have a separate container image repo for these images (more work -- less obvious value right now?)

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[hayleycd]

@segiddins @haydentherapper asked me to tag you in this PR. Let me know what you think, I am happy to make changes.

[segiddins]

Coming back to this is on my todo list, once we have some more usage of the client

[haydentherapper]

@segiddins Would you be OK with merging this now?