unprivileged/rootless composefs

[cgwalters]

One concern that's come up but we haven't formally tracked is that we can't do "native composefs" (as an EROFS) from an unprivileged context.

There are two overall options:

• Add a system service; I sketched this out in https://github.com/cgwalters/composefs-oci-experimental?tab=readme-ov-file#mounting-composefs-as-non-root and I believe it could be safe.
• Use FUSE (Alex mentioned this, and also a relatively recent big change here is FUSE passthrough landed. Although it itself may not yet work in a userns? Need to look.

Of these I believe the system service should be our default stance; I think we can design it so it's quite low risk to the system.