Feature Request: AdvLinkMTU setting on Router Advertisements Page

[linuxgemini]

Important notices
Before you add a new report, we ask you kindly to acknowledge the following:

• I have read the contributing guide lines at https://github.com/opnsense/core/blob/master/CONTRIBUTING.md

• I have searched the existing issues and I'm convinced that mine is new.

Is your feature request related to a problem? Please describe.
Using GIF interfaces through a VPN tunnel that has lower MTU than all other interfaces in OPNsense (like WireGuard) causes some SSL websites to not load (the usual MTU issue). This can be (usually) mitigated by lowering the advertised MTU on radvd. However on dhcpd_radvd_configure

core/src/etc/inc/plugins.inc.d/dhcpd.inc

Lines 250 to 261 in 2a216f9

	$mtu = legacy_interface_stats($realif)['mtu'];
	if (isset($config['interfaces'][$dhcpv6if]['track6-interface'])) {
	$realtrackif = get_real_interface($config['interfaces'][$dhcpv6if]['track6-interface'], 'inet6');
	$trackmtu = legacy_interface_stats($realtrackif)['mtu'];
	if (!empty($trackmtu)) {
	if ($trackmtu < $mtu) {
	$mtu = $trackmtu;
	}
	}
	}

the MTU is derived from the radvd parent/target interface (and maybe from 6rd tracked interface that is mentioned on #2643 and #2553)

Describe the solution you'd like
Addition of AdvLinkMTU to

core/src/www/services_router_advertisements.php

Line 40 in 45b0d68

$advanced_options = array('AdvDefaultLifetime', 'AdvValidLifetime', 'AdvPreferredLifetime', 'AdvRDNSSLifetime', 'AdvDNSSLLifetime', 'AdvRouteLifetime');

and re-calculating the MTU in dhcpd.inc based from the config.

Describe alternatives you've considered
I am currently lowering the LAN MTU, however this now causes instability issues on few IPv4 sites.

Additional context
The reason I'm doing this "tunnel in a tunnel" is due to me having a dynamic IPv4 address and my VPN mesh in WireGuard is not IPv6 oriented. Plus the server side of the tunnel does not have an update mechanism for IP updates.

[linuxgemini]

#4064 is merged and modified on aef9ada