This helper function takes a value returned by the query provider
(or loaded from disk, or supplied by query feeding), allocates
it in an arena if requested by the arena_cache modifier, and
then returns an erased copy of it.
This type alias specifies the type returned from query providers and the type
used for decoding. For regular queries this is the declared returned type V,
but arena_cache will use <V as ArenaCached>::Provided instead.