[v3] Support getProdiverData()

[pedro-ribeiro]

FirebaseUser exposes a getProviderData method that returns a list of objects that represent the linked identities of the user. This method is not yet supported by Firestack.

See firebase documentation.

[auser]

Not yet. Wanna try a PR? Should be pretty straight-forward to implement.

[Salakar]

This will sorted on v3 branch and can be accessed vua firestack.auth().currentUser.providerData as per the web api. The js code is done, just needs native side implementing