Controls for available CPU platforms

[iximeow]

#8728 added CPU platforms to instances and the API to set them, but any instance can have any (or no) CPU platform set on it. You can set instances to amd_turin when there are no Turin sleds around; this will effectively produce an instance that can't be started. You can set instances to amd_milan when there are no Milan sleds, which is needlessly restrictive (if you are a new customer in five years, I'm thinking about you!)

I think conceptually a list of allowed families that lives alongside quotas is probably most fitting? in an ideal world maybe even:

• an API to list available CPU families and hardware
• an (same?) API that includes CPU lineage information (what CPU platform is a subset of what)
  • if you're in the future after Turin, and disable Turin because you know you're in the future, we should probably suggest or just also disable Milan as it's even older

[iximeow]

(I went ahead and included release notes because I don't think this will come in the same release as #8728 and whenever it does come it seems good to give a heads up)

[iximeow]

tangential to CPU platforms, but probably similar in mechanism: as of #9385 one can make a whole slate of instances larger than any Gimlet can ever run. if there was a way to constrain available CPU platforms, it might make sense to have constraints on allowed VM shapes to go with it.

[david-crespo]

API endpoints to list available CPU families would be very useful in the console, e.g., for not making it seem like someone can make a 254 CPU VM in a rack that is all Gimlets.