New tag to allow plugin managers to opt into cache clears

Plugin managers utilize caching to prevent reparsing of their plugin definitions on every request.
The CachedDiscoveryInterface allows external code to clear their caches, but there needs to be a way for calling code to clear the caches of all plugin managers at once, like during module installation.

Any plugin manager that has a service ID starting with plugin.manager. will be automatically included in this list.

If a plugin manager has an custom service ID that does not meet that pattern, it can use the plugin_manager_cache_clear tag:

  entity.manager:
    class: Drupal\Core\Entity\EntityManager
    arguments: ['@container.namespaces', '@service_container', '@module_handler', '@cache.cache', '@language_manager', '@string_translation']
    tags:
      - { name: plugin_manager_cache_clear }