mgr/dashboard: unable to edit pipe config for bucket level policy of a bucket

mgr/dashboard: unable to edit pipe config for bucket level policy of a bucket

Fixes: https://tracker.ceph.com/issues/68387
Fixes Includes:
1) Passing additional parameter for 'user' and 'mode' as the user can be either system/dashboard or other values while creating pipe.
2) Previously while removing the src/dest bucket field, we were getting same old values on editing pipe, but now it will become '*' if empty value passed from frontend.

Signed-off-by: Naman Munet <namanmunet@li-ff83bccc-26af-11b2-a85c-a4b04bfb1003.ibm.com>
(cherry picked from commit 5a1a21573b92113144060e8475778c669b1de4aa)