Refactor gRPC API

[ccicconetti]

Split proto file in services + messages.

Rename package agent_api -> edgeless_api.

[raphaelhetzel]

I actually thought of splitting across the different services (to limit the interfaces a dev working on a subset needs to read), but that might not be needed.

[ccicconetti]

I actually thought of splitting across the different services (to limit the interfaces a dev working on a subset needs to read), but that might not be needed.

Yeah, I thought a lot about that option, too. Then I realized that:

1. for now the services are very compact (even with comments)
2. splitting the services is trivial and it does not have any impact on crates other than edgeless_api

So basically we can do further splitting as the API grows.