Facets summary should be cacheable, in case facets are using cacheable source.

Problem/Motivation

In the issue Facets summary block always cached the FacetsSummaryBlock has been marked uncacheable with the UncacheableDependencyTrait. According to that issue, the summary was always cached and the solution to that was to make the block cacheable in it's entirety "for now" in 2016. Fast forward to now and this issue has never been resolved.

The uncacheability of this block caused issues on our Varnish cache, especially since `X-Drupal-Dynamic-Cache` ends up as `UNCACHEABLE` and therefor our purge implementation will not pass through the `Cache-Tags`.

Steps to reproduce

Add a Facets summary block to the page, note that the entire page is uncacheable.

Proposed resolution

When reverting the patch that has been added in the aforementioned issue, I can't seem to reproduce the reported issue. I suggest to re-introduce the `url.query` context and make the block cacheable again.

Remaining tasks

• Collect cache metadata from active facets while rendering facets summary.
• Add corresponding facet summary cache tags into the Search API query, reuse Facet approach with ::registerFacet()

• Provide tests that will cover enabled cache scenarios.
  

  I think this can be achieved by creating new test class and implementing following tests from \Drupal\Tests\facets_summary\Functional\IntegrationTest and \Drupal\Tests\facets_summary\Functional\HierarchicalFacetIntegrationTest, but without UI part:

  IntegrationTest:

  1. testFramework - admin UI test, can be omitted
  2. testShowSummary
  3. testViewsCacheDisable, should cover none Search API cache plugins.
  4. testCount
  5. testBlockDelete - admin UI test, can be omitted
  6. configureShowCountProcessor - admin UI test, can be omitted
  7. configureResetFacetsProcessor - admin UI test, can be omitted
  8. testEmptyFacetLinks
  9. testResetFacetLink
  10. testResetFacetLink

  HierarchicalFacetIntegrationTest

  1. testHierarchicalFacet

  As alternative: change Facet source to cacheable view display and add $this->drupalLogin($admin) for the UI parts, and before results check switch back to anonymous, if needed: $this->drupalLogout()

User interface changes

N/a

API changes

Data model changes

Introduce:public Drupal\facets_summary\FacetsSummaryManager::returnBuiltFacet() which can be used in the getCacheTags, getCacheContext, getCacheMaxAge facet summary block.