fix: handle SkuNotAvailable provisioning error

[tallaxes]

Description

Handle SkuNotAvailable provisioning error, representing out of capacity for Spot, by taking the SKU out of consideration for 1hr. (In case for some reason this is observed for on-demand - should not happen, see below - it will still defensively take SKU out of consideration, but for 23hrs.)

Based on https://aka.ms/azureskunotavailable, it can happen if the SKU is either not available for a location or zone, or out of capacity for Spot. We only expect to observe the Spot case, not location or zone restrictions, because:

• SKUs with location restriction are already filtered out via sku.HasLocationRestriction, see fix: support non-zonal regions #35. (Originally SkuNotAvailable was observed prior to this filter).
• Zonal restrictions are filtered out internally by sku.AvailabilityZones, and don't get offerings

How was this change tested?

• make presubmit, manual testing (have not yet observed Spot out-of-capacity)

Does this change impact docs?

• Yes, PR includes docs updates
• Yes, issue opened: #
• No

Release Note

[Bryce-Soghigian]

LGTM, please remove SKUNotAvailable into azure-sdk-for-go-extensions when you get a chance.

Also it might be nice to add some test functions for response errors in extensions as well.