wireguard: add peers roaming support

Type: feature

With this change, peers are able to roam between different external
endpoints. Successfully authenticated handshake or data packet that is
received from a new endpoint will cause the peer's endpoint to be
updated accordingly.

Signed-off-by: Alexander Chernavin <achernavin@netgate.com>
Change-Id: Ib4eb7dfa3403f3fb9e8bbe19ba6237c4960c764c