Add auth providers linking/unlinking to <firebase-auth>

[FluorescentHallucinogen]

Add link/unlink (multiple) auth providers to an account feature to <firebase-auth> element.

For more info see: https://firebase.google.com/docs/auth/web/account-linking.

• [ ] Add linkWithPopup method.
• [ ] Add linkWithRedirect method.
• [ ] Add linkWithCredential method.
• [ ] Add unlink method.

[FluorescentHallucinogen]

@tjmonsi PTAL.

[tjmonsi]

@FluorescentHallucinogen thanks for the headsup. I'll check up on it.

[khammami]

@tjmonsi @FluorescentHallucinogen the user object returned by firebase-auth include already these methods

[FluorescentHallucinogen]

@khammami But these methods are not listed in https://webcomponents.org/element/firebase/polymerfire/elements/firebase-auth#methods. They should be declared in <firebase-auth> web component explicitly.