Create Rust oak_loader binary

[tiziano88]

We have the starting point of an oak runtime Rust library, but we do not have the entry point of a Rust binary, i.e. a counterpart to dev_oak_runner but in Rust.

[tiziano88]

@ipetr0v perhaps you could look into this, it should be similar to the work you did when splitting the dev and asylo loader.

[daviddrysdale]

cf. #540 which covers the Rust runtime library

[ipetr0v]

Should we continue using protobuf config files or switch to a something new (JSON, YAML etc.) ?
Related to #689

[tiziano88]

A large chunk of the config files will still be the serialized wasm modules, and for that I think protobuf is the one that makes more sense. JSON or YAML would presumably require to base64 encode them as binary data, which does not seem that great.

[ipetr0v]

Also, if I understand correctly gRPC client nodes are currently not created by the Rust Runtime, right?