Add action name to Probes casts
Add action name to Probes casts

This adds an atom (:register, :unregister) specifying the action
for the cast to perform, instead of relying on the argument count.

appsignal/appsignal-elixir,appsignal/appsignal-elixir,appsignal/appsignal-elixir,appsignal/appsignal-elixir