Add support for footer and customFormArea to LoginForm

[mvysny]

Describe your motivation

Vaadin 24.2.0.rc2: Only LoginOverlay has the support to have footer and customFormArea, yet LoginForm does not. This sounds like a strange omission, given that LoginOverlay and LoginForm are almost identical in other aspects.

Describe the solution you'd like

Is there a reason for this functionality to be omitted in LoginForm? If not, could you please add support for it?

Describe alternatives you've considered

No response

Additional context

No response

[web-padawan]

Is there a reason for this functionality to be omitted in LoginForm?

The main reason for not supporting this in LoginForm is that vaadin-login-form web component does not use Shadow DOM and therefore does not support slot. So we can't provide proper HTML API for custom content slots.

In case of LoginOverlay, we have slots and then custom logic to "teleport" the content into the login form.

[rolfsmeds]

Although the implementation and WC API will need to be different, we could certainly investigate how we could provide the same functionality for non-overlay login-form.