[Enhancement] Added wireguard support.

[dreirund]

I have added a wireguard-connection:

/usr/lib/network/connections/wireguard:

#!/bin/bash

. /usr/lib/network/network
: ${WIREGUARD:=wg-quick}

[[ -z "$WG_CONFIG" ]] && WG_CONFIG='/etc/wireguard/default.conf'

wireguard_up() {
  load_profile "$1"

  "${WIREGUARD}" up "${WG_CONFIG}"

  if [[ $? -ne 0 ]]; then
    report_fail "wireguard connection failed."
    exit 1
  fi
}

wireguard_down() {
  load_profile "$1"

  "${WIREGUARD}" down "${WG_CONFIG}"

  if [[ $? -ne 0 ]]; then
    report_fail "wireguard disconnection failed."
  fi
}

wireguard_$1 "$2"
exit $?

/etc/network.d/examples/wireguard:

CONNECTION="wireguard"
# WIREGUARD="wg-quick" # Optional; specifies the `wg-quick` compatible executable to use. If not specified, defaults to `wg-quick`.
INTERFACE="wg-ai"
WG_CONFIG="/etc/wireguard/wg-ai.conf" # Specify the wireguard configuration to use.

Depends on wireguard-tools and, of course, wireguard support in the kernel.

Fell free to incorporate. I don't know how to do pull requests, so I post it here that way.