Pre-built proto files

[rkusa]

Using dcs-grpc-stubs as a dependency in other Rust-based projects is a bit inconvenient as it requires this workaround to get it to build using its bundled protoc.

An alternative might be to pre-built the porto files instead of having them as part of the normal build process. Here is an example of another project doing something like this: https://github.com/open-telemetry/opentelemetry-rust/pull/881