[TASK] add WireGuard

[AiyionPrime]

Description

This implements basic support for WireGuard in accordance of the fastd-related paragraphs in code.

Motivation and Context

We, @freifunkh are currently building up our WireGuard infrastructure;
and giving back something to yanic after using it for years seemed appropriate.

Furthermore this resolves #183 and could bring new momentum to the discussion of #66.

Checklist:

• My code follows the code style of this project.
• I have added also tests for my new code.

I do not think fastd is documented either; so I'm unsure whether this is matter to document, or not.

• My change requires a change to the documentation.
• I have updated the documentation accordingly.

replaces #187 in order to use correct branches

[AiyionPrime]

please use pointer, here - it is nullable

Originally posted by @genofire in #187 (comment)

Done:
https://github.com/FreifunkBremen/yanic/pull/198/files#diff-66149b40837eb55dbca243a386f20f0f7e6355c2c634731dbe6b6c2d603361c7R83

[AiyionPrime]

We spoke about this yesterday in hackint#ffhb:
This is currently blocked by freifunk-gluon/gluon#2163 and freifunk-gluon/gluon#2168.

[AiyionPrime]

Update:
This is currently blocked by freifunk-gluon/gluon#2168.

[AiyionPrime]

This PR is currently not explicitly reflecting gluon-mesh-vpn-wireguards statistics-provider.
https://github.com/freifunk-gluon/gluon/blob/98664fdcb4dc90f5be33e8c9e17b87ffa4cfb80b/package/gluon-mesh-vpn-wireguard/src/respondd.c#L204

It basically wraps ubus call wgpeerselector.wg_mesh status, which returns:

{
	"peers": {
		"sn03": false,
		"sn02": false,
		"sn01": false,
		"sn10": false,
		"sn04": false,
		"sn09": false,
		"sn06": false,
		"sn07": false,
		"sn08": false,
		"sn05": {
			"established": 2635
		}
	}
}

Which in turn is put below statistics.mesh_vpn.
Yanic does currently have groups or peers there:

yanic/data/statistics.go

Line 51 in 4a9089b

Groups map[string]*MeshVPNPeerGroup `json:"groups,omitempty"`

As the wireguard-statistics-struct mimics the fastd-response, this should be fine.
I think this is good to go. @lemoer do you have objections?

[AiyionPrime]

@lemoer and I just agreed, this looks about right.

[AiyionPrime]

@genofire Please let me know when you find time to look over this; preferably before Wednesday noon, then I'll be gone for a week.

[genofire]

On first look, there is missing statistics

[genofire]

open issue for it #206 - @AiyionPrime maybe you could make an PR for this too