Build e2e tests for grpc server

[mbore]

Currently, we have only unit tests for grpc feature and need to test the generated code manually against the grpc server. We should be able to implement the following testing flow: write endpoints' definitions -> generate proto file -> generate client code -> write testing client calls & assertions Once the code is written, we could generate the proto file & client's code as part of the CI pipeline and use it in the test step.