ipn/ipnlocal: parse priority out of suggest-exit-node capability

When the `experimental:exit-node-steering` attribute is enabled for a
node, it is likely that the priority score for an exit node is
different when considering it as a suggested exit node versus
considering it for the routers / connectors that it hosts.

In order to distinguish between these two, the network map can now
contain the priority score for suggesting the exit node in the
`suggest-exit-node` capability, in that peer’s CapMap:

	"CapMap": {
		"suggest-exit-node": [
			{
				"Priority": 42
			}
		]
	}

Updates tailscale/corp#31011

Signed-off-by: Simon Law <sfllaw@tailscale.com>

tailcfg/traffic.go

@@ -0,0 +1,13 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package tailcfg
+
+type SuggestExitNode struct {
+	// Priority is the relative priority of this exit node. Nodes with a
+	// higher priority are preferred over nodes with a lower priority, nodes
+	// of equal probability may be selected arbitrarily. A priority of 0
+	// means the exit node has no a priority preference and a negative
+	// priority is not allowed.
+	Priority int `json:",omitempty"`
+}