mgr, qa: add `pending_modules` to asock command

mgr, qa: add `pending_modules` to asock command

Now, the command `ceph tell mgr mgr_status` will show a
"pending_modules" field. This is another way for Ceph operators
to check which modules haven't been initalized yet (in addition
to the health error).

This command was also added to testing scenarios in the workunit.

Fixes: https://tracker.ceph.com/issues/71631
Signed-off-by: Laura Flores <lflores@ibm.com>
(cherry picked from commit 68221661b00f8a6bef0fbd7b5401aa49eb5118d0)