Make section library template entities fieldable

I'm attaching a patch that does the most minimal job of making section library templates fieldable. It adds no new routes and uses the entity collection route as the base for managing the field UI. It adds no new classes or menu items and does not add fields to section_library's custom "add to library" forms.

This also doesn't include an update hook for sites that already have section_library installed; that would be a good enhancement.

However, with this patch in a site installing section_library for the first time, the following are true:

• Add additional fields to section templates at /admin/content/section-library/fields
• Manage the edit form display and the edit form (which is more content entity vanilla) will display all the fields as you have configured them.

Users could alter the add to library forms using route subscribers to define their own form classes or with form alter hooks to add fields as desired to those forms.

Future enhancements could include:

• Altering the add to library forms so that they automatically include the custom fields
• Adding an entity type update hook for sites already using section_library

The latest MR also converts the image field to a true image field which allows for applying image form options for example. Note: this is another change that requires an entity update.