Add ``trionics.maybe_open_context()`` an actor-scoped async multi-task
context manager resource caching API.

Adds an SC-safe cacheing async context manager api that only enters on
the *first* task entry and only exits on the *last* task exit while in
between delivering the same cached value per input key. Keys can be
either an explicit ``key`` named arg provided by the user or a
hashable ``kwargs`` dict (will be converted to a ``list[tuple]``) which
is passed to the underlying manager function as input.