feat: Make protoc dependency optional

[nathanwhit]

This PR makes the dependency on prost-build (and therefore protoc) optional.

To do this, the rust source generated by protoc is copied in-tree, and the build script will only recompile .proto files if the build_protos feature is enabled.

This does introduce a potential point of confusion if the proto definitions are modified, but presumably these are rarely changed.

[losfair]

Thanks, looks good to me - could you add a CI test that regenerates protoc output and verifies they are identical to the committed files?

[nathanwhit]

Thanks, looks good to me - could you add a CI test that regenerates protoc output and verifies they are identical to the committed files?

Added!