Generate the code using gogofaster and the grpc plugin

[dmathieu]

This changes the generation command so it uses gogofaster_out, as well as the gRPC plugin for all proto files.

Doing so generates the Unmarshal method for service payloads, which is required to unmarshal protobuf data send through an HTTP endpoint (which someone wishing to implement an OTLP endpoint through HTTP will have to do).

This change should also make it possible to use this repository within the collector instead of the current submodule.

Note: I didn't commit the regenerated code, because the license header gets dropped. So I suppose something, but I didn't see anything about it in RELEASING.md.

[MadVikingGod]

The collector is actively moving away from gogo: open-telemetry/opentelemetry-collector#4258

[bogdandrutu]

@MadVikingGod we are not actively moving away, we are looking into options :)