Documentation on minimal permissions required for FCM push notifications

[mclark-ably]

I think it would be ideal to document or make reference to the minimum required permissions needed for an FCM service account a customer will configure on the Ably dashboard to use the cloud messaging API https://firebase.google.com/docs/projects/iam/permissions#messaging

Most customers will want to follow the principle of least privilege when creating service accounts

See internal conversation

https://ably-real-time.slack.com/archives/CUNV4AV2Q/p1725532182525069?thread_ts=1725463134.620479&cid=CUNV4AV2Q