mgr/dashboard: fix error while accessing roles tab when policy attached

mgr/dashboard: fix error while accessing roles tab when policy attached

Fixes https://tracker.ceph.com/issues/64270

Issue:
======
Accessing Object->Users-Roles tab causing 500 internal servor error.
This is due to the "PermissionPolicies" which are attached to role and
backend was not handling this field for rgw roles.

Fix:
====
Added "PermissionPolicies" as the valid field in backend and updated
frontend to render the attached policy in formatted JSON

Signed-off-by: Afreen <afreen23.git@gmail.com>