Add msize configuration for windows-side 9P client (=higher WSL2 disk speed)

There is a continuous effort to improve performance of 9P connections in WSL2. If we can resolve it at least for bulk transfers - half of the problem would be gone. Key to this msize configuration on 9P client. On linux side msize is configurable in fstab, but it seems to be not configurable in windows 9P client.

Suboptimal msize might reduce transfer speed for large file transfer (currently I see 400Mb/s through 9P down from native 4.5Gb/s achievable under Linux in WSL2)

**Solution**
Registry config is added for 9P client on windows side. At least msize is configurable.

**Describe alternatives you've considered**
Increase of existing hardcoded msize to 1-32Mb.
Automatic msize? Or more documentation how 9P client works / configured.

**Additional context**
Impact of msize to performance: https://wiki.qemu.org/Documentation/9psetup#Performance_Considerations_.28msize.29
Here others set default values to 1-32Mb : https://github.com/lxc/lxd/issues/10219

Start by locating the Windows-side 9P client and the existing hardcoded msize setting. Review how this component reads Windows configuration, then determine how an msize registry setting should be exposed and validated. Done means the client accepts the configured value and bulk-transfer performance can be checked against the current default.

Written by the indexing model from the issue text.