bundle_of definition added to EntityType annotation to automatically generate Field UI manage operations to entity operations

Using a configuration entity to manage bundles of a fieldable content entity is a common pattern in Drupal 8. A new bundle_of property has been added, which defines this relationship.

Currently, the only thing that relies on that definition is field_ui.module, which uses it to add the following operations to the configuration entity list: Manage fields; Manage form display and Manage display.

The following entity types use bundle_of:

1. node_type for node
2. taxonomy_vocabulary for taxonomy_term
3. custom_block_type for custom_block
4. contact_category for contact_message

Example:

/**
 * Defines the Node type configuration entity.
 *
 * @EntityType(
 *   id = "node_type",
 *   label = @Translation("Content type"),
 *   module = "node",
 *   ...
 *   config_prefix = "type",
 *   bundle_of = "node",
 *   ...
 * )
 */
class NodeType extends ConfigEntityBase implements NodeTypeInterface {