Explore: Add the ability to upgrade the control plane only (pause worker nodes) to the console

Goal
Add the ability to choose between a full cluster upgrade or control plane upgrade (which will pause all worker pools) in the console.

Background
Currently in the console, users only have the ability to complete a full cluster upgrade. For many customers, upgrades take longer than what their maintenance window allows. Users need the ability to upgrade the control plane independently of the other worker nodes. 

Ex. Upgrades of huge clusters may take too long so admins may do the control plane this weekend, worker-pool-A next weekend, worker-pool-B the weekend after, etc.  It is all at a pool level, they will not be able to choose specific hosts.

Questions
What happens when the admin selects the control plane option and the control plane upgrade is complete. What happens to the worker and infra nodes? Do users have to take explicit action to upgrade those? Are there new alerts that we would need to account for?

From Scott: Worker pools can remain paused as long as is normally safe, which means until certificate rotation becomes critical which is at about 60 days. The admin would be advised to unpause them in order to complete the full upgrade but there's a future where they may remain paused in order to "skip" part of the upgrade when upgrading across multiple minors. Components like the apisever will be responsible for setting conditions which inhibit upgrades that are unsafe in those scenarios using the existing Upgradeable=False condition which the console now displays starting in 4.9 I believe, I need to actually look at that UX.

Design

1. Add the ability to choose between a cluster upgrade and a control plane upgrade
2. provide in context help information as to what the difference between those two choices are
3. come up with a way a clear way to display that when the control upgrade is in place that the worker nodes are paused
4. design the interaction for initiating the upgrades for each individual pool
5. create an alert to let users know they are approaching the 60 day window and advise them to complete the rest of the upgrade 

Additional Resources: 

• https://docs.google.com/presentation/d/1snT-whNTNvwllUZySSLJ7HdzTjqVsHNJlDv8jtR_dhw/edit#slide=id.gb6b9665c60_0_23
• The deck above, describes going from 4.N to 4.N+2 while the pools are paused which wasn't covered in the epic I mentioned though it's a natural extension. They'd click the "Upgrade Control Plane" while running 4.N, then click it again when it gets to 4.N+1, then unpause the pools once they get to 4.N+2.