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

Following up on #43: I now think #2749865-52: Rename the 'user.permissions' context to 'user.roles.permissions', to ensure correct cache context optimization → can be closed because of the comment I linked. Meaning this issue is no longer blocked by it nor by 📌 Add a container parameter that can remove the special behavior of UID#1 Fixed . Having said that, it no longer makes sense to fold either user.roles into user.permissions or vice versa (as explained in the other issue), so we should definitely not add that as an example in the documentation.

Tried to investigate the failing test. There's an issue with bubbling of that stuff. Within VariationCache::set(), that cache context is correctly optimized away and the cache tag added, but the information doesn't bubble up because RenderCache::set() doesn't do anything with the cacheability object. and I think it should. But it seems to work in the other test method, just not the node within the test entity.