[FR] Abillity to connect auth and storage emulator without env variables

**Is your feature request related to a problem? Please describe.**
I want to make a copy of my production env to develop locally, for this purpose, i need to copy data from firestore and some blob from storage.
To connect the firestore sdk locally, you can do
```javascript
admin.firestore().settings(...)
```
For other products like auth or storage, this kind of settings is not possible and you must provide an environment variable.
But all `app` initialized share this env variables and won't allow you to connect to both production and emulator environment

**Describe the solution you'd like**
Provide an interface like the `settings` of firestore for other emulator products

**Describe alternatives you've considered**
- Override `baseUrl` and `apiEndpoint` for my current storage instance
- Use scripts to export/import my production data