Fix: support WireGuard link-local remotes with ifindex specifier

[mvitale1989]

Netplan currently rejects Wireguard link-local remotes, if they specify an interfaces index.
For instance, setting endpoint: "[fe80::5054:ff:fe75:5b1f%enp1s0]:51820" on a peer fails with the following error:

# netplan apply
/run/netplan/10-lexnet.yaml:33:19: Error in network definition: invalid endpoint address or hostname '[fe80::5054:ff:fe75:5b1f%enp1s0]:51820'
        endpoint: "[fe80::5054:ff:fe75:5b1f%enp1s0]:51820"
                  ^

This is a bug as these are valid IPv6 addresses, and wireguard itself accepts them without any issue:

# wg
interface: wg0
  public key: REDACTED
  private key: (hidden)
  listening port: 51820

peer: REDACTED
  endpoint: [fe80::5054:ff:fe75:5b1f%enp1s0]:51820
  allowed ips: ::/0, 0.0.0.0/0
  latest handshake: 22 seconds ago
  transfer: 360 B received, 280 B sent
  persistent keepalive: every 10 seconds

This PR introduces support for an arbitrary ifindex string, within a Wireguard remote endpoint specification.

I wasn't yet able to file this as a bug on Launchpad as I'm not receiving the e-mail verification to create an account, but I'm opening the PR already as I had the fix ready.

Checklist

• Runs make check successfully.
• Retains code coverage (make check-coverage). (the new lines are covered)
• New/changed keys in YAML format are documented. (no YAML format changes)