Add new Entry Types field

[leevigraham]

The immediate answer to this might be to just use a matrix with max entries set to one.

I'm thinking that there are use cases where you may want a Link entry type that has a URL field and some other customisations like an icon field. This could be created as a new entry type.

The only way to add this to an element in the field layout designer is to create an associated Links matrix.

I wonder if a field that allows a single Entry Type could simplify this.

Thinking a bit more broadly there's already an Addresses field that operates like a matrix but with a single entry type assigned.

[brandonkelly]

Similar to #15626 and #14600