Handling RedisCache is not connected.

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

https://github.com/StackExchange/StackExchange.Redis/issues/1120
https://github.com/StackExchange/StackExchange.Redis/issues/1322

Currently when there is an exception in connection, it throws an exception

https://github.com/dotnet/extensions/blob/3196f53061fcbe12fb2ebe81834124710d822a8d/src/Caching/StackExchangeRedis/src/RedisCache.cs#L173-L192

and during get

https://github.com/dotnet/extensions/blob/3196f53061fcbe12fb2ebe81834124710d822a8d/src/Caching/StackExchangeRedis/src/RedisCache.cs#L240

It will throw an exception as well.

### Describe the solution you'd like

I think we should try to establish the connection during initialization and provide an event for connection status. So we can somehow check if Redis cache is available before using it.