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/urls.go

@@ -28,7 +28,7 @@ func (c Client) FetchURLList(ctx context.Context, config model.OOAPIURLListConfi
 		query.Set("category_codes", strings.Join(config.Categories, ","))
 	}
 	var response urlListResult
-	err := c.Client.GetJSONWithQuery(ctx, "/api/v1/test-list/urls", query, &response)
+	err := c.APIClient.GetJSONWithQuery(ctx, "/api/v1/test-list/urls", query, &response)
 	if err != nil {
 		return nil, err
 	}