Allow override the default *.googleapis.com hostname when calling API

## What feature would you like to add

Allow override the default *.googleapis.com hostname when calling API.
For well-know reason the *.googleapis.com is inaccessible in some place, however developers can host reverse proxy under their own domain name to workaround that.

## How would you use it?

Add a method to allow developer customize the hostname when calling firebase apis that are hosted under googleapis.com.
e.g.
firebasestorage.googleapis.com