Add support for Redis Clusters in RedisSessionInterface

[thebmw]

[shinxi]

Hi @Lxstr, could we consider merging this PR?

We're using AWS ElastiCache with Redis Cluster, but flask-session currently attempts to connect to a local Redis instance by default. As a result, we're seeing the following error when Flask calls

redis.exceptions.ConnectionError: Error 111 connecting to localhost:6379. Connection refused.

This PR resolves the issue by enabling flask-session to work properly with Redis Cluster. Thanks!