Add support for associating a subnet with shared network via remote-subnet4-set

The cb_cmds hooks library supports remote-subnet4-set command which can be used to add or update a subnet in the config backend. There is currently no way to add a subnet to an existing shared network, nor update the subnet being in a shared network. The only way to add a subnet within a shared network is by issuing remote-network4-set, but that's an overkill if the shared network contains many subnets.

The following is the relevant part of the design: https://gitlab.isc.org/isc-projects/kea/wikis/designs/configuration-in-db-design#remote-subnet4-set

It proposes an optional parameter shared-network-name within a subnet definition which would specify that the added/updated subnet should be placed within a shared network having a given name. This issue is about implementing this functionality.