rgw: utilize is_impersonating for forwarded sts requests

rgw: utilize is_impersonating for forwarded sts requests

With the introduction of is_impersonating in SysReqApplier,
RoleApplier can now use the same mechanism to mark when a request
has been forwarded by a system user on behalf of another role (e.g.,
through STS) to mark it as a system request (s->system_request).

Signed-off-by: Seena Fallah <seenafallah@gmail.com>