Allow rendering entity references with custom elements

took a fist stab on this. it basically works already, but the ajax submission of the configuration form does not work yet. probably the entity-ce-display form has some problem there still.

@fago I did a fix, please check. I/you can merge after.

Last commit link below == https://git.drupalcode.org/project/custom_elements/-/merge_requests/60/d...

(I have a pre-implementation of a "flatten" option that I may or may need, since yesterday evening, but I'm keeping it out of this MR on purpose, for simplicity.

Will ask, while changing more tests.)

I figured the cache metadata was missing, added that and merged!

Automatically closed - issue fixed for 2 weeks with no activity.