sd-dhcp-server: Add Hostname= option to static leases

sd-dhcp-server: Add Hostname= option to static leases

This adds a new `Hostname=` option to the [DHCPServerStaticLease]
section in .network files, allowing an administrator to assign a
specific hostname to a client receiving a static lease.

We automatically select the correct DHCP option to use based on the
format of the provided string:

- Single DNS labels are sent as Option 12.
- Names with multiple DNS labels are sent as Option 81 in wire format.

Fixes: #39634