fix(firebase): add ability to pass in firebase config

[chanceaclark]

☕️ Reasoning

Adds the ability to pass in Firebase config into the adapter to initialize the app, rather than initializing by default. This is useful if you've initialized the app on the client and want to reuse the config on the server. Alongside that, adds some documentation on how to use it.

Note: Ran prettier --write packages/adapter-firebase which cleaned up some files.

🧢 Checklist

• [ ] Documentation
• [ ] Tests
• [ ] Ready to be merged

🎫 Affected issues

Fixes: #4927

📌 Resources

• Contributing guidelines
• Code of conduct
• Contributing to Open Source

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Name	Status	Preview	Updated
next-auth	✅ Ready (Inspect)	Visit Preview	Sep 2, 2022 at 6:54PM (UTC)

[chanceaclark]

cc. @balazsorban44

[itsmylife]

@chanceaclark @balazsorban44 @ndom91 @lluia Sorry for pinging you all but will we have approval soon?