Deprecate Descriptor UID/GID

[tri-adam]

type Descriptor struct contains UID/GID values:

sif/pkg/sif/sif.go

Lines 332 to 333 in 08f0698

	UID int64 // system user owning the file
	GID int64 // system group owning the file

These are populated when a data object is created by calling user.Current:

sif/pkg/sif/create.go

Lines 51 to 69 in 08f0698

	// Get current user and returns both uid and gid.
	func getUserIDs() (int64, int64, error) {
	u, err := user.Current()
	if err != nil {
	return -1, -1, fmt.Errorf("getting current user info: %s", err)
	}
	uid, err := strconv.Atoi(u.Uid)
	if err != nil {
	return -1, -1, fmt.Errorf("converting UID: %s", err)
	}
	gid, err := strconv.Atoi(u.Gid)
	if err != nil {
	return -1, -1, fmt.Errorf("converting GID: %s", err)
	}
	return int64(uid), int64(gid), nil
	}

UID/GID values are by their nature system specific, and thus these seem to be of limited utility in a container format that is meant to support mobility. I cannot find a single use of these in SingularityCE, nor any other projects using the github.com/sylabs/sif module.

I suggest we deprecate these in github.com/sylabs/sif. In github.com/sylabs/sif/v2, I suggest we set these values to zero, and remove them from the exported API.