osd: Add method to store config option key/value on the MON store

osd: Add method to store config option key/value on the MON store

Add method mon_cmd_set_config() to save config option key and
value to the MON store. The ConfigMonitor command, 'config set' is
used to achieve this.

A corresponding get method is unnecessary since any config option
found on the MON store is loaded during OSD boot-up and set using
the md_config_t::set_mon_vals() method. Therefore, the existing
versions of ConfigProxy::get_val() method are sufficient to get
the latest value for the config option.

Fixes: https://tracker.ceph.com/issues/51464
Signed-off-by: Sridhar Seshasayee <sseshasa@redhat.com>