wgengine{,/wgint}: add wgint.Peer wrapper type, add to wgengine.Engine

This adds a method to wgengine.Engine and plumbed down into magicsock
to add a way to get a type-safe Tailscale-safe wrapper around a
wireguard-go device.Peer that only exposes methods that are safe for
Tailscale to use internally.

It also removes HandshakeAttempts from PeerStatusLite that was just
added as it wasn't needed yet and is now accessible ala cart as needed
from the Peer type accessor.

None of this is used yet.

Updates #7617

Change-Id: I07be0c4e6679883e6eeddf8dbed7394c9e79c5f4
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

wgengine/wgengine.go

@@ -11,11 +11,13 @@ import (
 	"tailscale.com/ipn/ipnstate"
 	"tailscale.com/net/dns"
 	"tailscale.com/tailcfg"
+	"tailscale.com/types/key"
 	"tailscale.com/types/netmap"
 	"tailscale.com/wgengine/capture"
 	"tailscale.com/wgengine/filter"
 	"tailscale.com/wgengine/router"
 	"tailscale.com/wgengine/wgcfg"
+	"tailscale.com/wgengine/wgint"
 )
 
 // Status is the Engine status.
@@ -84,6 +86,10 @@ type Engine interface {
 	// away, sent to the callback registered via SetStatusCallback.
 	RequestStatus()
 
+	// PeerByKey returns the WireGuard status of the provided peer.
+	// If the peer is not found, ok is false.
+	PeerByKey(key.NodePublic) (_ wgint.Peer, ok bool)
+
 	// Close shuts down this wireguard instance, remove any routes
 	// it added, etc. To bring it up again later, you'll need a
 	// new Engine.