Cannot use template string with wildcard as value_field for relation widget

[vnourdin]

Describe the bug
When using a template string that contains a * wildcard (to access every item of a list) as value_field of a relation widget, the widget says "No options".

To Reproduce

1. Create a list widget, with fields a and b, and populate it
2. Create a relation widget that points to the previous list
3. Set value_field to 'mylist.*.a'
4. It works
5. Set value_field to 'a is {{mylist.*.a}} - and b {{mylist.*.b}}'
6. It says "No options"

Expected behavior
Template string should work even with wildcard.

[vnourdin]

Initial discussion in #3712 (comment), generating the field in the list collection with preSave event is a workaround.