Define the initial shipper gRPC interface

[cmacknz]

Define the initial shipper gRPC service and use protoc to generate the client and server implementations. The interface should be defined in the https://github.com/elastic/elastic-agent-client repository to ensure the entire elastic agent interface is defined in one place, and can be imported without depending on it's internal implementation.

The proposal specifies an initial event protocol. Decide whether specification should be taken as the starting point, or we can start with a minimal version of it (a Publish RPC with empty request and response messages) and add to it as we add features tot he shipper.

The shipper gRPC client and server should be an importable Go package once this issue is complete.

[fearful-symmetry]

With the first skeleton now merged, how much of this is still open? Do we want to move the gRPC code itself to the client repo?

[cmacknz]

Let's close this. I think our life may be easier with the proto files defined in the shipper repository as we iterate on them. We can move them later.