Issue #2935392 by catch: EntityTypeInterface is overly prescriptive about...

   * should be used for them. Where possible, link relationships should use

   * established IANA relationships rather than custom relationships.

   *

   * Every entity type should, at minimum, define "canonical", which is the

   * pattern for URIs to that entity. Even if the entity will have no HTML page

   * exposed to users it should still have a canonical URI in order to be

   * compatible with web services. Entities that will be user-editable via an

   * HTML page must also define an "edit-form" relationship.

   * Entities which can be viewed should define "canonical", which is the

   * pattern for URIs to that entity including REST. Entities that will be

   * user-editable via an HTML page should define an "edit-form" relationship.

   *

   * By default, the following placeholders are supported:

   * - [entityType]: The entity type itself will also be a valid token for the