rust_prost_library doesn't seem to permit controlling the crate name

I'm slowly heading down the path I raised [here](https://github.com/bazelbuild/rules_rust/discussions/2283) and this is the first concrete bug - it seems like rust_prost_library derives the crate name from the proto it is for. This causes a problem if we're building with two or more different protoc configurations (and yes I realise that means they end up as different incompatible crates, but thats really no different than feature unification, and a problem that should be fairly obvious to folk choosing to combine rust and bazel).