tunneldigger: add L2TPv3 tunneling

[PolynomialDivision]

tunneldigger-broker: add broker for tunneldigger

In mesh communities, tunneldigger is widely used to create L2TPv3 tunnels
and mesh via them. Since the broker is typically installed on other
distributions, the openwrt broker package has not received any
maintenance in recent years [0]. I  take now care of the further maintaince
of this package. Furthermore, I consulted with the maintainers to ensure
that they were comfortable with the change [1].

This PR is just a refactoring of the already existing opkg package from
wlanslovenija. It fixes config parsing and in general the config, adapts
to the new python syntax and fixes dependency handling.

- [0] https://github.com/wlanslovenija/firmware-packages-opkg/tree/master/net/tunneldigger-broker
- [1] https://github.com/wlanslovenija/firmware-packages-opkg/issues/24

tunneldigger: add package for establishing L2TPv3 tunnels over UDP

In the previous commit we already added tunneldigger-broker. Add the
corresponding client.

This PR is just a refactoring of the already existing opkg package from
wlanslovenija [0].

[0] - https://github.com/wlanslovenija/firmware-packages-opkg/tree/master/net/tunneldigger

Maintainer: me
Compile tested: mt7621
Run tested: mt7621

[PolynomialDivision]

CI fails because of some issue that is not directly related with the packages:

 Collected errors:
 * pkg_hash_check_unresolved: cannot find dependency libxtables12 for tc-mod-iptables
 * pkg_hash_fetch_best_installation_candidate: Packages for tc-mod-iptables found, but incompatible with the architectures configured
 * pkg_hash_check_unresolved: cannot find dependency libxtables12 for tc-full
 * pkg_hash_fetch_best_installation_candidate: Packages for tc-full found, but incompatible with the architectures configured
 * satisfy_dependencies_for: Cannot satisfy the following dependencies for tunneldigger-broker:
 * 	libxtables12
 * opkg_install_cmd: Cannot install package tunneldigger-broker.

[1715173329]

lgtm ;)

[PolynomialDivision]

lgtm ;)

Thanks a lot for your review. :)