Add optional derivers for menu links.

[Jaesin]

The goal of this project has always been to minimize the technical requirements to create a fully capable content entity. This is another task in the spirit of removing complexity for the most basic use case while maintaining a path for extensible entities.

The goad is to add derivers so the following link yamls are optional:

1. foo.links.action.yml
2. foo.links.task.yml
3. foo.links.menu.yml

[Jaesin]

See: https://www.drupal.org/node/2122253

example.local_tasks:
  deriver: 'Drupal\example\Plugin\Derivative\DynamicLocalTasks'
  weight: 100

The deriver should be optional for a given entity because an entity might want to override the menu items. They should check for a local_task_provider entity handler that would supply the local tasks. The default provider should be in the content_entity_base module.

See #50

[Jaesin]

Something like:

 *   handlers = {
 *     "link_provider" = {
 *       "action" = "\Drupal\content_entity_base\Entity\Link\EntityActionLinkProvider",
 *       "menu"   = "\Drupal\content_entity_base\Entity\Link\EntityMenuLinkProvider",
 *       "task"   = "\Drupal\content_entity_base\Entity\Link\EntityTaskLinkProvider",
 *     },