AppSignal reporter

[grzuy]

appsignal elixir package will no longer support automatic exception tracking per what was discussed in appsignal/appsignal-elixir#1017 (comment).

Given that is exactly what tower is good at, automatically/easily report all errors, it might be good to provide a tower_appsignal reporter package that provides this for users as a replacement or complement to appsignal.

[grzuy]

@martosaur FYI ☝️ 👋 🙂