refactor: UI tweaks

[russellwheatley]

• Initial commit added a ScrollView to the AuthPickerView due to it's height 2c4dd76 (#1255)
• Second commit was an attempt to refactor Phone Auth to a button in an attempt to declutter AuthPickerView. af59d88 (#1255)
• Third commit - rename Facebook button label to "Sign in with Facebook" to keep naming consistent with other providers ("Sign in with..."). bf20f0c (#1255)

To give you context on Phone Auth, the UI was rendering directly in the AuthPickerView:

The aim was to make it Phone Auth a button so it would make AuthPickerView less cluttered and closer to previous implementation:

The problem we have now is - how do we shift the View to Phone Auth? It has to happen from within Core Auth package (AuthPickerView). See comment for further details: https://github.com/firebase/FirebaseUI-iOS/pull/1255/files#r2088761197

I guess we should bear in mind this is for alpha release at the moment.

[russellwheatley]

This is the nub of the problem and it is the same problem we have rendering the buttons. Core auth package doesn't know about PhoneAuthView so how can we render it?

We could:

1. Use AnyView again like with rendering buttons.
2. Move Phone Auth package in to Core Auth package - A couple hundred lines of code extra, not a tremendous amount.
3. An alternative solution???

[morganchen12]

To match behavior from old FirebaseUI, this should be a button that launches the phone auth view flow in a modal view. I think if we can we should keep it a separate package despite its small size since it's less popular than email auth.

[russellwheatley]

I've updated to present a modal, I've had to follow a similar pattern to renderingButtons() logic for the same reason discussed previously (i.e. using AnyView): dea04f6 (#1255)