Improve caching of the custom processor

Problem/Motivation

I tried to enable caching for my views that uses facets and the module pretty path implementation. In results I got max-age 0 when exporting config.

After deep dive into the caching mechanism for views I found that one of the facet processors is uncacheabe.

The solution is to provide UnchangingCacheableDependencyTrait.

I tested this locally and after resaving views the max-age was -1 which is permanent cache.