Add test for generated service stub

- Create one simple test for generated Service stubs in preparation
for making more changes in this area.
- Add dev dependency on pytest-asyncio in order to use ChannelFor
from grpclib.testing more easily.
- Create a new example proto containing a minimal rpc example.

betterproto/tests/service.proto

@@ -0,0 +1,15 @@
+syntax = "proto3";
+
+package service;
+
+message DoThingRequest {
+  int32 iterations = 1;
+}
+
+message DoThingResponse {
+  int32 successfulIterations = 1;
+}
+
+service ExampleService {
+  rpc DoThing (DoThingRequest) returns (DoThingResponse);
+}