runtime/rpc: Use more go-codec friendly datastructures

[Yawning]

The Go worker needs to be able to make sense of WorkerLocalRPCCallResponse messages received from the enclave. While it is possible (with some difficulty) to parse out unit enums serialized by serde_cbor with go-codec, it's unpleasant and rather ugly.

The various message types defined in runtime/src/rpc/types.rs should be changed to have struct variants so that they are easier/cleaner to interact with on the Go side.

[kostko]

This was fixed together with #1885.