Add interface and use trait to make the config service a cacheable dependency

[adam-vessey]

... really, just inheriting cachability from the config at which it points.

GitHub Issue: Somewhat auxiliary to Islandora/documentation#2134

What does this Pull Request do?

Views dependent on the config behind the service can more easily be given the dependency on the config.

What's new?

Make the \Drupal\openseadragon\Config service implement the \Drupal\Core\Cache\CacheableDependencyInterface and return appropriately.

• Does this change require documentation to be updated? No.
• Does this change add any new dependencies? No.
• Does this change require any other modifications to be made to the repository
  (i.e. Regeneration activity, etc.)? No.
• Could this change impact execution of existing code? Unlikely.

How should this be tested?

Presently, has no observable outcome; however, this will allow for cached items to be appropriately invalidated when the config changes.

Additional Notes:

Any additional information that you think would be helpful when reviewing this
PR.

Interested parties

Tag (@ mention) interested parties or, if unsure, @Islandora/8-x-committers

[jordandukart]

Looking a-ok to me.

[seth-shaw-asu]

👍