Change FCM argument setting to accept any term for the auth name

[mononym]

Is your feature request related to a problem? Please describe.
We have a use case where we handle multiple google applications and the notifications sent to them. Each one has its own unique Goth worker to provide the auth settings, and so we configure multiple Goth workers/FCM dispatchers.

Pigeon forces the auth name for Goth to be an atom.

When reading from our configuration files we take the names for these applications and read them in and create new goth workers for each of these application names. Which means we're forced to convert strings to atoms. While it is possible to do this "safely", it's not an ideal setup to deal with.

Describe the solution you'd like
Goth accepts any term for the name of its workers because it uses Registry to register/look them up.

I would like the FCM args checker to not be so strict in forcing atoms where it isn't required.