support current mainline Linux kernel features (eg bcachefs)

**Is your feature request related to a problem? Please describe.**

When I want to try recent kernel features (e.g. bcachefs), I have to go back to "some real VM" because WSL needs additional patches.

Currently the wsl patches are out-of-tree. They are non-trivial to integrate into a current kernel release and it's non-trivial to build the wsl2 base image (based off such a recent kernel).

**Describe the solution you'd like**

* provide a **mainline-release+wsl patchset** kernel, with wsl2 ready kconfig, ready to build & ship.
* provide a binary ready to use
* e.g. **enable wsl vendors like ubuntu and arch and openSUSE** to pick that up and offer it as an alternative kernel
* or if you want to control it, offer it as a **wsl2-current-kernel stream for "wsl.exe --update"**
* note that mature Linux distros have a **fallback mechanism**, a wsl.exe --update solution should have the same

just to spell it out: **"additionally" mainlining the patches** will help for two reasons:

* technically, the majority of forward porting is done by those changing APIs you consume.
* politically, mainlining enforces the statement: "we, MS, are dead serious about wsl."

**Describe alternatives you've considered**

I don't see any.

**Additional context**

none.

No files, tests, or implementation entry points are named. Start by reviewing the existing out-of-tree WSL patches and the process for building the WSL2 base image, then determine the scope of a current-kernel patchset, configuration, binary distribution, and fallback update stream; done means a concrete, maintainable plan accepted by the WSL and kernel stakeholders.

Written by the indexing model from the issue text.