Make client.Context implement gRPC ClientConn (#6342)

* Add QueryConn to client.Context

* Add integration test

* imports

* spelling

* Make client.Context implement grpc ClientConn

Co-authored-by: Alexander Bezobchuk <alexanderbez@users.noreply.github.com>
Co-authored-by: Federico Kunze <31522760+fedekunze@users.noreply.github.com>

codec/testdata/test_service.go

@@ -0,0 +1,19 @@
+package testdata
+
+import (
+	"context"
+	"fmt"
+)
+
+type TestServiceImpl struct{}
+
+func (e TestServiceImpl) Echo(_ context.Context, req *EchoRequest) (*EchoResponse, error) {
+	return &EchoResponse{Message: req.Message}, nil
+}
+
+func (e TestServiceImpl) SayHello(_ context.Context, request *SayHelloRequest) (*SayHelloResponse, error) {
+	greeting := fmt.Sprintf("Hello %s!", request.Name)
+	return &SayHelloResponse{Greeting: greeting}, nil
+}
+
+var _ TestServiceServer = TestServiceImpl{}