luks2: Add idx member to struct grub_luks2_keyslot/segment/digest

luks2: Add idx member to struct grub_luks2_keyslot/segment/digest

This allows code using these structs to know the named key associated with
these json data structures. In the future we can use these to provide better
error messages to the user.

Get rid of idx local variable in luks2_get_keyslot() which was overloaded to
be used for both keyslot and segment slot keys.

Signed-off-by: Glenn Washburn <development@efficientek.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>