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/measurementmeta.go

@@ -54,7 +54,7 @@ func (c Client) GetMeasurementMeta(
 		query.Add("full", "true")
 	}
 	var response MeasurementMeta
-	err := (httpx.Client{
+	err := (&httpx.APIClient{
 		BaseURL:    c.BaseURL,
 		HTTPClient: c.HTTPClient,
 		Logger:     c.Logger,