Implement interop test server

[hawkw]

List from https://github.com/grpc/grpc/blob/master/doc/interop-test-descriptions.md#server

• [x] EmptyCall
• [x] UnaryCall
• [ ] CacheableUnaryCall
• [ ] CompressedResponse
• [ ] CompressedRequest
• [ ] StreamingInputCall
• [ ] StreamingOutputCall
• [x] FullDuplexCall
• [x] Echo Status
• [ ] Echo Metadata
• [ ] Observe ResponseParameters.interval_us
• [x] Unimplemented Method
• [x] Unimplemented Service

[hawkw]

Some of the tests will depend on #2.

[hawkw]

Currently using [the test server from grpc-go: interop-server-go-linux-amd64.zip

[seanmonstar]

Some server interop tests were implemented in https://github.com/tower-rs/tower-grpc/pull/123. I'll update the top comment with a checklist.