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/psiphon.go

@@ -11,7 +11,9 @@ func (c Client) FetchPsiphonConfig(ctx context.Context) ([]byte, error) {
 	if err != nil {
 		return nil, err
 	}
-	client := c.Client
+	// Note: the following code is very bad: it copies the original
+	// API client and then overrides one of its fields. Bleah...
+	client := c.APIClient
 	client.Authorization = fmt.Sprintf("Bearer %s", auth.Token)
 	return client.FetchResource(ctx, "/api/v1/test-list/psiphon-config")
 }