Save authentication FCM config as JSON dumps

[norkans7]

Fixes https://textit.sentry.io/issues/5686387015/ as courier expects the config as a string and marshals that into JSON
https://github.com/nyaruka/courier/blob/d78749478c8a64ed865935d54771e61531ca48dc/handlers/firebase/handler.go#L257

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (92678a4) to head (b170c6b).

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #5433   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          507       507           
  Lines        25251     25252    +1     
=========================================
+ Hits         25251     25252    +1     

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[rowanseymour]

@norkans7 cn't we fix this on the courier side? marshaled JSON inside of JSON isn't ideal

[norkans7]

replaced by nyaruka/courier#769