[PP-1] Provide a "add" link template for the block_content_type entity type

Problem/Motivation

Noticed this while reviewing #2987964: Move custom block types admin link to admin/structure.

The block_content_type Entity Type does not provide an add link template in its entity type annotation:

 *   links = {
 *     "delete-form" = "/admin/structure/block/block-content/manage/{block_content_type}/delete",
 *     "edit-form" = "/admin/structure/block/block-content/manage/{block_content_type}",
 *     "entity-permissions-form" = "/admin/structure/block/block-content/manage/{block_content_type}/permissions",
 *     "collection" = "/admin/structure/block/block-content/types",
 *   },

Currently, the add route is provided in block_content.routing.yml and the route name does not follow the established pattern, block_content.type_add where it should be entity.block_content_type.add.

Proposed resolution

Provide a add link template in the block_content_type entity annotation.

Since block_content_type already uses the AdminHtmlRouteProvider this will automatically generate the entity.block_content_type.add route.

The block_content.type_add route can then be deprecated, this requires #3159210: Support route aliasing (Symfony 5.4) and allow deprecating the route name to be committed, so postponing on that issue.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet