feat: add agent acks to in-memory coordinator (#12786)

When an agent receives a node, it responds with an ACK which is relayed
to the client. After the client receives the ACK, it's allowed to begin
pinging.

tailnet/conn.go

@@ -88,7 +88,6 @@ type Options struct {
 	// falling back. This is useful for misbehaving proxies that prevent
 	// fallback due to odd behavior, like Azure App Proxy.
 	DERPForceWebSockets bool
-
 	// BlockEndpoints specifies whether P2P endpoints are blocked.
 	// If so, only DERPs can establish connections.
 	BlockEndpoints bool
@@ -311,6 +310,10 @@ type Conn struct {
 	trafficStats *connstats.Statistics
 }
 
+func (c *Conn) SetTunnelDestination(id uuid.UUID) {
+	c.configMaps.setTunnelDestination(id)
+}
+
 func (c *Conn) GetBlockEndpoints() bool {
 	return c.configMaps.getBlockEndpoints() && c.nodeUpdater.getBlockEndpoints()
 }