Add MetalLB in FRR mode via flags, add webhookvalidation disable option

[mowoe]

This PR adds the possibility to enable the MetalLB plugin in FRR mode. This is required for IPv6 Peering sessions.
While writing tests for this mode, i came across an issue with the validatingwebhookconfiguration, which is described here:
metallb/metallb#1597

As the maintainers of MetalLB say themselves, this issue is entirely unpredictable and occurs seemingly at random. This is kind of bad, especially for automated tests. Hence, I added a turn-off flag for the validation webhooks. This can be dangerous, as the manifests wont get sanity checked by metallb then, so i only added the possibility to turn this off via the flags and not via the prompt. This way, we can disable them during tests, while keeping the user from just disabling them without knowing what that actually means.

You can also take a look at this blogpost: https://blog.mowoe.com/building-a-multi-node-ipv6-enabled-bare-metal-kubernetes-cluster-with-microk8s-metallb-longhorn-and-vyos.html
to learn more about why frr-mode is handy.

[ktsakalozos]

Thank you for this PR @mowoe. It is useful and it would be a good addition. We need to hold it however for a bit. We try to move away from the argument passing using the addon:arg:arg:arg notation as it is rather confusing. We will have to return on this PR as soon as we refactor the metallb addon.

I appreciate your understanding.

[mowoe]

Hi @ktsakalozos, thank you for your reply!
I completely understand this decision and i have to say, i am glad, that you decided to abandon this way of argument passing, as i found it a bit tedious.
You can just get back to me here once you are finished with the transition and i will adjust my pr accordingly.