EvictingCacheMap extract

If you store a unique_ptr as a value in an EvictingCacheMap there is no way to take ownership of the object again. Could EvictingCacheMap support the extract interface similar to map (https://en.cppreference.com/w/cpp/container/map/extract).

The issue names EvictingCacheMap but no implementation file or test. Start by locating that container and its existing map-like ownership operations, then compare the requested behavior with std::map::extract. Done means a stored unique_ptr can be recovered through an extract interface, with tests covering ownership transfer.

Written by the indexing model from the issue text.