Add a GitHub Action to verify that PRs always contain the latest rendered configuration

[tpaschalis]

To avoid the proto definitions and rendered client code get out of sync, we should introduce a GitHub Action as a required check that makes running make buf-generate results in no changes.

[spartan0x117]

Looking into this, what is tricky is that the specific versions of protoc-gen-go and protoc-gen-connect-go can cause parts of the code to differ (here, for example). Should we instead add an action that does generation on the merge of a pull request?