Facet summary entity and SAPI Event subscriber code polishing.

Facet summary & Facet entity should have the same base class or trait. List of methods that has to be included in the base class or trait:

• get/set for facetSourceId
• loadProcessors
• getProcessorConfigs
• getProcessors
• getProcessorsByStage
• addProcessor
• removeProcessor
• getFacetSource
• calculateDependencies
• postSave, postDelete & clearBlockCache - is something that is currently missing in the Summary entity, but it has there to clear block deriver and plugin cache

We can even go further and do the same with entity interfaces.
Pros:

• More structured code base.
• Better test coverage: . I was wrong, the only thing that was missing is alterQuery.
• Decrease future copy/pasted code as it was done in https://www.drupal.org/project/facets/issues/3264284 →

Cons: Architectural changes of a module with this amount of downloads, will cause unexpected issues.

Same goes for the "facets_summary.search_api_subscriber" and "facets.search_api_subscriber" that suppose to alter SAPI query and add facet related information. Currently they are using different approach: Summary alters a query no matter what, facets on the other hand do the check whether this query supports facets. This is wrong because summary can't be built without facets information.
Proposed solution:
- Introduce one entry point, can be an abstract class with a query alter method that then will do the check and trigger corresponding manager method.