Fall back to preview feature band when there is no current workload manifest matching the current SDK

[marcpopMSFT]

During development, we want to be able to reversion for a new feature band without forcing an update to the workload.
Additionally, a workload may choose not to make changes and so may not want to be forced to update.

As part of this, we might want to design a system for knowing what workloads we expect so we can deprecate the workload and not load it.

[dotnet-issue-labeler]

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.