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/geolocate/ubuntu.go

@@ -20,7 +20,7 @@ func ubuntuIPLookup(
 	logger model.Logger,
 	userAgent string,
 ) (string, error) {
-	data, err := (httpx.Client{
+	data, err := (&httpx.APIClient{
 		BaseURL:    "https://geoip.ubuntu.com/",
 		HTTPClient: httpClient,
 		Logger:     logger,