Ability to customize token name / rename token to ably-token

[sacOO7]

• Currently, there is a popular laravel-jwt-library => jwt-auth that uses token as a default key name.
• For a given auth request, laravel-broadcaster responds with jwt-token containing channel claims. Client resends it for next request. This is used by server to check for existing channel claims and if token can be upgraded.
• The main issue here is token-key name sent by client in the payload. As per comment 401 Error on Subsequent Private/Public Channel Subscriptions #36 (comment), there is a need to modify this token name.

[sacOO7]

Current token name is token. I recommend to rename it to ably-token. There is no need to provide customization since it doesn't serve any utility as such !