CloudAPI: minimal interface for EPH and existing API server

[filiphaftek]

Part of

Contains basic methods:

service InstanceService {
  rpc ListInstances(ListInstancesRequest) returns (ListInstancesResponse) {}
  rpc CreateInstance(CreateInstanceRequest) returns (CreateInstanceResponse) {}
  rpc UpdateInstanceVersion(UpdateInstanceVersionRequest) returns (UpdateInstanceVersionResponse) {}
  rpc UpdateInstanceLease(UpdateInstanceLeaseRequest) returns (UpdateInstanceLeaseResponse) {}
  rpc UpdateInstanceFeatures(UpdateInstanceFeaturesRequest) returns (UpdateInstanceFeaturesResponse) {}
  rpc GetInstance(GetInstanceRequest) returns (GetInstanceResponse) {}
  rpc DeleteInstance(DeleteInstanceRequest) returns (DeleteInstanceResponse) {}
}

Sample usage with client - [code](https://github.com/sourcegraph/controller/pull/1363/files#diff-b6d948d5c8903c77acd5c2f1fbdac44ba1204adc7ce6ebcaed5ed18e384627dfR98