[FEATURE] Accept Widget type for footer, title, ...

[jthomaschewski]

Is your feature request related to a problem? Please describe. Currently it's difficult to do flexible customization of flutter_login. E.g. it's possible to provide a footer message as String but what if the developer wants to add a link/button to the footer?

This would be easy if footer and other "extension slots" would accept a Widget type instead of String. To get the old behavior one would just need to pass the String within a Text-widget.

Describe the solution you'd like

before:

LoginProvider(
  footer: "Some footer"
)

after:

LoginProvider(
  footer: Text("Some footer")
)

This would be a breaking change (maybe v3.x?). Alternatively the widget could be accepted in a separate property while keeping the old API intact:

LoginProvider(
  footerWidget: Text("Some footer")
)

Additional context Many feature requests are asking about adding functionality which can be either implemented with various feature flags/properties. But at least some of them could be covered by just adding some "slots" which can be filled with arbitrary widgets.

In general it would be nice to be able to do some more customization of various widgets provided by this package - in any way. Maybe by using a more modular approach with an easy way of constructing a custom flutter_login widget by reusing various components/controllers or any different valid approach.