[ISC-support #20152] Support for custom spaces in flex_option hook

https://support.isc.org/Ticket/Display.html?id=20152

A Support customer has requested that the flex_option hooks library allow editing of DHCP options in custom spaces. Currently, this only works for "dhcp4" or "dhcp6".

https://gitlab.isc.org/isc-projects/kea/-/blob/master/src/hooks/dhcp/flex_option/flex_option.cc#L127