Sometimes a library exposes a very complex API but uses a gen_server
under the hood, and we want to pool this gen_server, but we're not
supposed to explicitly do `gen_server:call/3` nor `gen_server:cast/2`,
but use the API instead. To enable this, we expose the possibility to
run a function callback with a worker once a worker has been found.

Co-authored-by: Brujo Benavides <elbrujohalcon@gmail.com>