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/login.go

@@ -29,7 +29,7 @@ func (c Client) MaybeLogin(ctx context.Context) error {
 	}
 	c.LoginCalls.Add(1)
 	var auth LoginAuth
-	if err := c.Client.PostJSON(ctx, "/api/v1/login", *creds, &auth); err != nil {
+	if err := c.APIClient.PostJSON(ctx, "/api/v1/login", *creds, &auth); err != nil {
 		return err
 	}
 	state.Expire = auth.Expire