dhcp-server-identifier in client class scope

I tried to configure dhcp-server-identifier in the client class scope but the server didn't send an ACK. Instead it dropped the DHCP request complaining that "it contains a foreign server identifier". After that, I found in the documentation that "this option is only supported at the global, shared network, and subnet levels; it must not be specified on the client class or host reservation levels".

I think that when it is not supported in client classes, Kea should complain during the validation of the configuration so that the users know right away that they shouldn't use the option in this scope.

Or even better, it would be great if you could add the support in this scope so that we could use dhcp-server-identifier for PXE boot the same way as we did in client groups in ISC DHCP.