mgr/cephadm: adding new cephadm service mgmt-gateway

mgr/cephadm: adding new cephadm service mgmt-gateway

adding mgmt-gateway, a new cephadm service based on nginx, to act as
the front-end and single entry point to the cluster. This gateway
offers unified access to all Ceph applications, including the
Ceph dashboard and monitoring tools (Prometheus, Grafana, ..),
while enhancing security and simplifying access management
through nginx.

Fixes: https://tracker.ceph.com/issues/66095
Signed-off-by: Redouane Kachach <rkachach@ibm.com>