Footer section for legal text on UI Components

[camhart]

Is your feature request related to a problem? Please describe.
Right now there's no easy way to include a custom footer within the UI components that could be used to support legal text (privacy policy, terms, etc.)

Describe the solution you'd like
Add an additional slot to sign up and sign in that can be customized to include legal text.

Describe alternatives you've considered
Right now I've placed it below the authentication box, but it feels/looks disconnected. The existing footer slots all replace necessary content/features because the components themselves use them.

[ericclemmons]

I'm thinking we can add a slot under the forms (outside the card), since this content should persist for all states.

[ericclemmons]

This is being published to the @next release via:

#695

For example – https://github.com/aws-amplify/amplify-ui/blob/main/examples/next/pages/ui/components/authenticator/custom-slots/index.page.tsx