EntityType link templates: using link relation type names as keys is a problem

Problem/Motivation

This is a problem introduced in #1970360: Entities should define URI templates and standard links and #2019123: Use the same canonical URI paths as for HTML routes.

I discovered and explained this in #2293697-124: EntityResource POST routes all use the confusing default: use entity types' https://www.drupal.org/link-relations/create link template if available at length. Relevant portion quoted here:

This made me realize: then how can you discern registered link relation types versus extension link relation types?! We should use identify them by their name and URI respectively. That'd match the spec. So we should continue to do it like this patch is already doing: using URIs!

However… that won't work because Drupal 8 shipped with violations of this principle already, and we can't change that because it'd break BC. We should have done #2113345: Define a mechanism for custom link relationships before D8 shipped, then we wouldn't have made this mistake. For example for Node in HEAD:

 *   links = {
 *     "canonical" = "/node/{node}",
 *     "delete-form" = "/node/{node}/delete",
 *     "edit-form" = "/node/{node}/edit",
 *     "version-history" = "/node/{node}/revisions",
 *     "revision" = "/node/{node}/revisions/{node_revision}/view",
 *   }

Of those, only canonical, edit-form and version-history are registered link relation types (i.e. registered with and accepted by the IANA). So that means delete-form and revision are already extension link relation types (i.e. Drupal-specific).

(This also means that if the IANA accepts new link relation types that happen to use the same names as the ones Drupal used, but with a different meaning, that we will be forced to break BC. For D9, we should change the entity type links annotation to use URIs as keys for extension link relation types, and we should also make link_relation_types.yml use URIs as keys for those.)

[…]

In fact, @linclark predicted that the logic in the REST module would have to change depending on how #2113345: Define a mechanism for custom link relationships would be solved. See her comment at #2019123-43: Use the same canonical URI paths as for HTML routes.

Proposed resolution

1. Update NodeViewController (after #2282029: Automate link relationship headers for all entity types, not just nodes it'll be EntityViewController that we need to update) to use the URI of an extension link relation type that #2113345: Define a mechanism for custom link relationships have now finally made available.
2. Update @EntityType annotations to use this URI as the key for links entries. So not:
   

    *   links = {
    *     "canonical" = "/node/{node}",
    *     "delete-form" = "/node/{node}/delete",
   

   but:

    *   links = {
    *     "canonical" = "/node/{node}",
    *     "https://drupal.org/link-relations/delete-form" = "/node/{node}/delete",
   

Point 1 can be done now, with minimal disruption.

Point 2 can only be done in Drupal 9, it'd be too disruptive.

Remaining tasks

TBD

User interface changes

None.

API changes

TBD

Data model changes

TBD