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>