So I want to store an
entity_type in a string field in my content entity.
Since the list of entities can change over time, I can't use a
list_string field with allowed options. I don't know if calling out to
entity_manager would even work properly in that context.
So my current plan just to use a
string field and somehow validate that the values the system attempts to save are valid entity type machine names.
How can I perform this validation at the field level? At the entity form level, I will obviously have to add a custom
select field and interpret/set the value in the
(entity form class)::save() method.