Support Hyper-V isolation

[Aziks0]

Description

Let me give you a bit of history:

• WCOW: buildkitd can't spin up Hyper-V isolated containers for RUN commands #5682 was opened, asking the same thing I'm asking now.
• [POC] wcow: option to run builds in hyperv isolation #5753 was opened, using an ENV to set the executor's isolation to Hyper-V.
• Support HyperVIsolation option in containerd executor in Windows #6224 got merged and superseded [POC] wcow: option to run builds in hyperv isolation #5753, closing it and it's related issue, although the added HyperVIsolation field is... never set anywhere 😅. It's used in moby, I guess.
• Can not specify hyperv isolation mode in buildctl #6986 was opened, asking the same thing I'm asking now (see the pattern?).
  • A "HOLY what kind of LLM bullshit is that?" response was given but I'll give you the AI;DR: build buildkitd yourself. Hmm, ok? Thanks for the answer. The 48 steps were really necessary.
  • The issue got closed.

This feels like a running gag at this point.

Anyway, here's the request, one more time: support Hyper-V isolation.

Adding an --isolated or --isolation=hyperv flag to buildctl would be quite nice, as proposed before. But adding the flag to buildkitd instead would be the easiest, I think, because that's when the executor gets configured and runs (I'm not familiar enough with the codebase to know for sure).