feat: [firebase_ui_auth] [firebase_ui_oauth] Allow for custom labels to be used for custom OAuth providers

[Rexios80]

Description

I made a custom OAuth provider so that I could add it to the sign in page. The issue is that the existing code expects there to only ever be the providers that exist in this repo. This was causing a crash when fetching the OAuth button label text.

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[Rexios80]

Seems like there are analysis issues in other packages unrelated to this PR. Probably not within the scope of this PR to fix them.

[lesnitsky]

@Rexios80 out of curiosity: what provider did you implement? Is this an officially supported provider that is available on firebase console?

[Rexios80]

https://github.com/IO-Design-Team/firebase_ui_oauth_oidc

[Rexios80]

Do you want to own that package? Or is it too custom to be in this repo? I was going to publish it myself if you don't see an issue with it.

[lesnitsky]

@Rexios80 feel free to publish