Feature Request: have plugin fetch protos

Add functionality to fetch protos from the repo. Optimally, without downloading an executable. In a heavily secure environment, downloading the buf executable is gonna get dinged for security audit. Having the ability to have this would require an https curl-like call to get the specific proto This will help with generating protos on an already existing proto build that can't easily convert to buf.build's buf CLL, but still allow the project to utilize Buf.build's powerful registry. I'd be happy to take a stab at this, it looks doable..