mgr/cephadm: Add snmp-gateway service support

mgr/cephadm: Add snmp-gateway service support

Add a new snmp-gateway service to provide a bridge between
Prometheus and an SNMP management platform. The gateway
service uses https://github.com/maxwo/snmp_notifier to provide
an SNMP v2c and SNMP V3 support.

The SNMP V3 support mandates at least authentication, and also
offers authentication and privacy (encryption).

Fixes: https://tracker.ceph.com/issues/52920
Signed-off-by: Paul Cuzner <pcuzner@redhat.com>
(cherry picked from commit c2f5e105ca4870b2cb124db662537c20e6daadae)

Conflicts:
	src/pybind/mgr/cephadm/module.py
	src/pybind/mgr/orchestrator/_interface.py
	src/pybind/mgr/orchestrator/module.py
	src/python-common/ceph/deployment/service_spec.py