feat(ExperimentOrchestraClient): add CheckIn (#263)

We use ExperimentOrchestraClient in several places to help us
calling probe-services APIs. We need to call CheckIn because we
want to use CheckIn in InputLoader.

(We also want to remove the URLs API, but that is not something
doable now, since the mobile app is still using this API via
the wrappers at pkg/oonimkall.)

Work part of https://github.com/ooni/probe/issues/1299.

internal/engine/inputloader_test.go

@@ -80,6 +80,10 @@ func TestInputLoaderNewOrchestraClientFailure(t *testing.T) {
 
 type InputLoaderBrokenOrchestraClient struct{}
 
+func (InputLoaderBrokenOrchestraClient) CheckIn(ctx context.Context, config model.CheckInConfig) (*model.CheckInInfo, error) {
+	return nil, io.EOF
+}
+
 func (InputLoaderBrokenOrchestraClient) FetchPsiphonConfig(ctx context.Context) ([]byte, error) {
 	return nil, io.EOF
 }