This change allows using a thin volume as external origin for thin volume in another pool, and chaining such volumes.

Steps:

1. Create a thin volume lv1 in pool1 with external origin - either by converting a volume to thin-volume, or by taking a snapshot with external origin of inactive and RO volume

2. Use a thin volume as external origin in another pool - convert the lv1 LV to a thin volume in pool2

3. Chaining - Convert the lv1 to a thin volume in pool3

Existing Tests in lvm2 testsuite:

• https://gitlab.com/lvmteam/lvm2/-/blob/main/test/shell/lvconvert-thin-external-stack.sh?ref_type=heads
• https://gitlab.com/lvmteam/lvm2/-/blob/main/test/shell/lvcreate-thin-external-stack.sh?ref_type=heads

Man page changes:

• https://gitlab.com/lvmteam/lvm2/-/commit/901becc748fd035f1145081b1ce3c36efbcda4bf