Allow non-intrinsic (implementation-dependent) cache context services to specify their parents via DynamicParentContextInterface::getParents()

Quoting @effulgentsia in #2428703: Add a 'user.permissions' cache context (was: "Should cache contexts be able to associate a cache tag?"):

How about this to address #74: we embed the hierarchy in the context name / service ID only when that hierarchy is intrinsic to the meaning of the context, not merely an implementation choice.

[…]

In a followup, I think we can add a service tag to convey an implementation-driven parent relationship. For example:

  cache_context.pagers:
   ...
    tags:
      - { name: cache.context, parent: cache_context.url.query_args:page }
  cache_context.route:
    ...
    tags:
      - { name: cache.context, parent: cache_context.url }

And thereby allow CacheContexts::optimizeTokens() to optimize for implementations where pagers are solely determined by a single URL query argument and routing is done exclusively by URL.