common/config: Add methods to return the default value of a config option

common/config: Add methods to return the default value of a config option

Add wrapper method "get_val_default()" to the ConfigProxy class that takes
the config option key to search. This method in-turn calls another method
with the same name added to md_config_t class that does the actual work of
searching for the config option. If the option is valid, _get_val_default()
is used to get the default value. Otherwise, the wrapper method returns
std::nullopt.

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