[StackExchange.Redis] Add DefaultReadCommandFlag into RedisCacheOptions for read operations

Is your feature request related to a problem? Please describe.

I am trying to hookup RedisCache to multiple nodes in AWS ElastiCache. I have 1 primary node and n replication nodes. Currently when configuring RedisCacheOptions.ConfigurationOptions with multiple Endpoints, RedisCache doesn't read from the replication nodes.

Describe the solution you'd like

In StackExchange.Redis there is a CommandFlags enum that will allow us to PreferSlave. I would like to add another property along the lines of DefaultReadCommandFlag to RedisCacheOptions for read commands. This will allow us to specify PreferSlave and distribute load across the replication nodes.

Command Flags: https://github.com/StackExchange/StackExchange.Redis/blob/c72ffa8413572262b8eb9f8f4e0c5da07f856359/src/StackExchange.Redis/Enums/CommandFlags.cs#L27-L47