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/probeservices/bouncer.go

@@ -9,6 +9,6 @@ import (
 // GetTestHelpers is like GetCollectors but for test helpers.
 func (c Client) GetTestHelpers(
 	ctx context.Context) (output map[string][]model.OOAPIService, err error) {
-	err = c.Client.GetJSON(ctx, "/api/v1/test-helpers", &output)
+	err = c.APIClient.GetJSON(ctx, "/api/v1/test-helpers", &output)
 	return
 }