networkd/dhcpserver: Save and expose the client hostname sent when requesting a DHCP lease.

DHCP clients often send their own hostname in option 12. It can be useful
to store it with the lease so it can be shown to a human to easily identify
which lease belongs to which device.

RFC: https://www.rfc-editor.org/rfc/rfc2132#section-3.14

src/network/networkd-json.c

@@ -1191,6 +1191,7 @@ static int dhcp_server_offered_leases_build_json(Link *link, JsonVariant **ret)
                                                                lease->client_id.data,
                                                                lease->client_id.length),
                                                JSON_BUILD_PAIR_IN4_ADDR_NON_NULL("Address", &address),
+                                               JSON_BUILD_PAIR_STRING_NON_EMPTY("Hostname", lease->hostname),
                                                JSON_BUILD_PAIR_FINITE_USEC(
                                                                "ExpirationUSec", lease->expiration)));
                 if (r < 0)