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

test/fuzz/fuzz-network-parser/dhcp-server-static-lease

@@ -16,10 +16,12 @@ DNS=9.9.9.9
 [DHCPServerStaticLease]
 MACAddress=12:34:56:78:9a:bc
 Address=10.1.1.2
+Hostname=testhost
 
 [DHCPServerStaticLease]
 MACAddress=12:34:56:78:9a:bc
 Address=10.1.1.3
+Hostname=device.example.com
 
 [DHCPServerStaticLease]
 Address=10.1.1.4