feat: show tailnet peer diagnostics after coder ping (#12314)

Beginnings of a solution to #12297 

Doesn't cover disco or definitively display whether we successfully connected to DERP, but shows some checklist diagnostics for connecting to an agent.

For this first PR, I just added it to `coder ping` to see how we like it, but could be incorporated into `coder ssh` _et al._ after a timeout.

```
$ coder ping dogfood2
p2p connection established in 147ms
pong from dogfood2 p2p via  95.217.xxx.yyy:42631  in 147ms
pong from dogfood2 p2p via  95.217.xxx.yyy:42631  in 140ms
pong from dogfood2 p2p via  95.217.xxx.yyy:42631  in 140ms
✔ preferred DERP region 999 (Council Bluffs, Iowa)
✔ sent local data to Coder networking coodinator
✔ received remote agent data from Coder networking coordinator
    preferred DERP 10013 (Europe Fly.io (Paris))
    endpoints: 95.217.xxx.yyy:42631, 95.217.xxx.yyy:37576, 172.17.0.1:37576, 172.20.0.10:37576
✔ Wireguard handshake 11s ago
```

tailnet/node.go

@@ -36,6 +36,7 @@ type nodeUpdater struct {
 	addresses            []netip.Prefix
 	lastStatus           time.Time
 	blockEndpoints       bool
+	sentNode             bool // for PeerDiagnostics
 }
 
 // updateLoop waits until the config is dirty and then calls the callback with the newest node.
@@ -79,6 +80,7 @@ func (u *nodeUpdater) updateLoop() {
 		u.logger.Debug(context.Background(), "calling nodeUpdater callback", slog.F("node", node))
 		callback(node)
 		u.L.Lock()
+		u.sentNode = true
 	}
 }
 
@@ -212,6 +214,7 @@ func (u *nodeUpdater) setCallback(callback func(node *Node)) {
 	defer u.L.Unlock()
 	u.callback = callback
 	u.dirty = true
+	u.sentNode = false
 	u.Broadcast()
 }
 
@@ -228,3 +231,11 @@ func (u *nodeUpdater) setBlockEndpoints(blockEndpoints bool) {
 	u.blockEndpoints = blockEndpoints
 	u.Broadcast()
 }
+
+// fillPeerDiagnostics fills out the PeerDiagnostics with PreferredDERP and SentNode
+func (u *nodeUpdater) fillPeerDiagnostics(d *PeerDiagnostics) {
+	u.L.Lock()
+	defer u.L.Unlock()
+	d.PreferredDERP = u.preferredDERP
+	d.SentNode = u.sentNode
+}