Dnsmasq - add DHCP config field for default gateway

[nrauso]

In simple scenarios, you can use dnsmasq on NS8 to set up a basic DHCP server.
The current implementation assigns the NS8 IP as the default gateway for clients that obtain an address through DHCP.

Proposed solution
To prevent routing issues, it would be beneficial to add an extra field in the DHCP configuration interface, allowing the sysadmin to specify the correct default gateway to distribute to DHCP clients.

[Tbaile]

QA module: ghcr.io/nethserver/dnsmasq:dhcp-gateway-field

• Fresh install
  • Install a fresh instance of the module using the image above
  • Init the module, check that in DHCP section Gateway option is present
  • Try to break the save (leave it empty, incomplete or out of the defined network)
  • Set a correct value, check with a client in the same network that the gateway is set correctly (use nmap --script broadcast-dhcp-discover)
• Update
  • Update an existing module with the DHCP section enabled.
  • Check with nmap --script broadcast-dhcp-discover that the client still gets correctly the DHCP info
  • Open the DHCP section in the module, check that the gateway field is empty (correct, defaults to DNSMasq)
  • Check that the field is now required

[nrauso]

test case 1: VERIFIED
test case 2: VERIFIED