Add support for listening to Azure Maintenance Events

[amsoedal]

Adding an automatic subscription to the AzureRedisEvents pubsub channel for Azure caches. This channel notifies clients of upcoming maintenance and failover events.

By exposing these events, users will be able to know about maintenance ahead of time, and can implement their own logic (e.g. diverting traffic from the cache to another database for the duration of the maintenance event) in response, with the goal of minimizing downtime and disrupted connections. We also automatically refresh our view of the topology of the cluster in response to certain events.

Here are some of the possible notifications:

// Indicates that a maintenance event is scheduled. May be several minutes from now
NodeMaintenanceScheduled,

// This event gets fired ~20s before maintenance begins
NodeMaintenanceStarting,

// This event gets fired when maintenance is imminent (<5s)
NodeMaintenanceStart,

// Indicates that the node maintenance operation is over
NodeMaintenanceEnded,

// Indicates that a replica has been promoted to primary
NodeMaintenanceFailover

[amsoedal]

Hi @NickCraver, I closed #1865 and opened this one instead! Also working on addressing remaining comments from @mgravell. Big thanks to both of you!

[NickCraver]

I merged in an unrelated PR on the search side - updating to get those failing test runs off the radar.

[philon-msft]

@NickCraver and @mgravell , I believe all the planned changes are now complete. Feel free to merge if we're in agreement

[NickCraver]

I made one small tweak to avoid allocating the list every time on the azure endpoint check - looking good! Thanks for all the work and refinements here <3