Kea: support shared networks

[giggio]

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Is your feature request related to a problem? Please describe.

I need to have the ability to have 2 subnets on the same link.

Describe the solution you like

Kea already supports this as shared networks. The concept does not yet exist in OpnSense. It could be a simple text string that names the shared network, but I suspect this will not be enough as Kea support grows.

For example, you can set the interface name for the shared network (as you can for the subnet), but this is not yet available in OpnSense, but I suspect it will eventually be requested and added.

For this reason, it might be more interesting to create the concept of shared network, give it a name, and eventually set the interface.

This could be all classified under an advanced configuration.

Describe alternatives you considered

Not using OpnSense to control Kea.

Additional context

See Kea docs on shared networks.