refactor(httpx): improve and modernize (1/n) (#647)

This PR starts to implement the refactoring described at https://github.com/ooni/probe/issues/1951. I originally wrote more patches than the ones in this PR, but overall they were not readable. Since I want to squash and merge, here's a reasonable subset of the original patches that will still be readable and understandable in the future.

internal/engine/experiment/webconnectivity/control.go

@@ -53,7 +53,7 @@ type ControlResponse struct {
 func Control(
 	ctx context.Context, sess model.ExperimentSession,
 	thAddr string, creq ControlRequest) (out ControlResponse, err error) {
-	clnt := httpx.Client{
+	clnt := &httpx.APIClient{
 		BaseURL:    thAddr,
 		HTTPClient: sess.DefaultHTTPClient(),
 		Logger:     sess.Logger(),