Commit: Add composefs digest and sig to the commit metadata

Commit: Add composefs digest and sig to the commit metadata

If `composefs-apply-sig` is enabled (default no) we add an
ostree.composefs digest to the commit metadata. This can be verified
on deploy.

This is a separate option from the generic `composefs` option which
controls whether composefs is used during deploy. It is separate
because we want to not have to force use of fs-verity, etc during the
build.

If the `composefs-certfile` and `composefs-keyfile` keys in the
ex-integrity group are set, then the commit metadata also gets a
ostree.composefs-sig containing the signature of the composefs file.