[feature] Add sentinel support to ActionCable Redis subscription adapter
When I tried to deploy it on to Kubernetes, however, I ran into a small stumbling block because to deploy Redis on to Kubernetes in a highly available manner it seems the recommended way is through use of Redis Sentinels:
redis-rails already supports sentinels, so use of Redis with sentinels for cache, sessions, etc. is not a problem: https://github.com/redis-store/redis-rails#usage-with-redis-sentinel
However, the ActionCable Redis subscription adapter is configured independently, and when I dived into the code it became apparent that the configuration did not currently support specifying Redis sentinel configuration (even though the underlying redis gem powering both ActionCable and redis-rails already does).
It looks like it should only take a couple of lines of additional code at https://github.com/rails/rails/blob/master/actioncable/lib/action_cable/subscription_adapter/redis.rb#L13 to support an additional `sentinels: [...]` configuration parameter (if present).
I realize that I could simply supply my own custom lambda to ActionCable::SubscriptionAdapter::Redis.redis_connector in an initializer to support sentinels there, but am wondering if this is something the rest of the community might appreciate. If so, I'll gladly raise a PR.