Moved proto deps into toolchain to avoid confusing boiler plate code

[UebelAndre]

Users looking to register their own toolchain sometimes stumble over the fact that every use of rust_grpc_library or rust_proto_library now requires them to also add the appropriate targets to the rust_deps attributes. This is easily missed and not necessary. Instead, these dependencies should be added to the @rules_rust//proto:toolchain being used so there's a single source of truth for all the dependencies required by these rules.

Additionally, this allows users to optionally not register the default toolchain in cases where they want to define their own but would like to ensure other dependencies are brought in.

closes #483

[UebelAndre]

This is a reboot of #519

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[illicitonion]

Seems pretty reasonable :)