Streamline metadata after GoogleSignIn removal

[mokagio]

This PR does some tidy up:

• Make LoginFieldsMeta internal so that we can more easily modify it in the future
• Restructures SocialService into SocialUser, removing the SocialServiceName SocialService enum cases duplicity
• Uses SocialUser instead of SocialService in the presentSignupEpilogue WordPressAuthenticatorDelegateProtocol method

The rationale for those changes was to improve encapsulation and remove duplication. I hope it'll help us further simplify the library in the future.

You can see the code in action in:

• https://github.com/woocommerce/woocommerce-ios/pull/10483
• https://github.com/wordpress-mobile/WordPress-iOS/pull/21344

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• [x] I have considered if this change warrants release notes and have added them to the appropriate section in the CHANGELOG.md if necessary.

[mokagio]

@crazytonyli this is ready for another look at your earliest convenience. Thanks