Add a post-cache-load hook

Migrated issue, originally created by Wichert Akkerman (wichert)

It would be practical if there was a way to detect that data was returned from the cache and then do some extra post-processing. My particular use case is caching SQLAlchemy ORM instances which needs to be merged into the session. Currently I am doing something like this:

:::python
@region.cache_on_arguments()
def _expensive_function(foo, bar):
    ...

def expensive_function(foo, bar):
    result = _expensive_function(foo, bar)
    if result not in session:
        session.merge(result, load=False)
    return result

This could be reworked as a decorator to be more generic and easier to read, but it still has to do this in-session check and possible merge on every invocation instead of only for cache hits. It would be nice to be able to do this:

:::python
def _merge(obj):
    session.merge(obj, load=False)

@region.cache_on_arguments(on_cache_hit=_merge)
def _expensive_function(foo, bar):
    ...

Start by locating the Python implementation of cache_on_arguments and tracing how cache hits are handled. Define the callback behavior and verify it with the relevant cache decorator tests; done should mean the hook runs only after cached data is returned without changing normal misses.