Lease service: SendManifest

[andrewhare]

Implements SendManifest from akash-network/akash-api#130.

Resolves akash-network/support#191

A few other things that are in this:

• A simple gRPC test harness. @troian mentioned that there are other gRPC tests, but I used this to familiarize myself with all the different interfaces behind the service, how they interact, and what they actually call.
• I refactored the mTLS logic into a single function. Eventually, the REST interface will be gone, but while we maintain both servers, we won't have multiple implementations to maintain as well.
• A small refactor to the grpc.Serve function to make it easier to test.
• I broke out the implementation of each proto service into its own file and its own struct. The main server struct composes these.