derp, wgengine: let clients advertise an opaque app name to DERP servers

Add an AppName field to the DERP ClientInfo so DERP servers can
attribute connections to the application making them, primarily for
best effort stats purposes. The value is plumbed per engine instance
rather than via a process global, so a process hosting multiple stacks
can attribute each one's DERP connections separately:
wgengine.Config.DERPAppName flows through magicsock.Options and
derphttp.Client into the naclbox-sealed ClientInfo JSON. Old servers
ignore the unknown field.

There are no callers in the tree yet setting the name.

Updates tailscale/corp#24454

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Change-Id: Ia7d3e9c2b6f8140e5a9d7c3b2e6f1a8d4c0b5e9f

derp/derphttp/derphttp_client.go

@@ -60,6 +60,7 @@ type Client struct {
 	DNSCache      *dnscache.Resolver // optional; nil means no caching
 	MeshKey       key.DERPMesh       // optional; for trusted clients
 	IsProber      bool               // optional; for probers to optional declare themselves as such
+	AppName       string             // optional; opaque app name to advertise to the server for stats
 
 	// WatchConnectionChanges is whether the client wishes to subscribe to
 	// notifications about clients connecting & disconnecting.
@@ -413,6 +414,7 @@ func (c *Client) connect(ctx context.Context, caller string) (client *derp.Clien
 			derp.MeshKey(c.MeshKey),
 			derp.CanAckPings(c.canAckPings),
 			derp.IsProber(c.IsProber),
+			derp.AppName(c.AppName),
 		)
 		if err != nil {
 			return nil, 0, err
@@ -558,6 +560,7 @@ func (c *Client) connect(ctx context.Context, caller string) (client *derp.Clien
 		derp.ServerPublicKey(serverPub),
 		derp.CanAckPings(c.canAckPings),
 		derp.IsProber(c.IsProber),
+		derp.AppName(c.AppName),
 	)
 	if err != nil {
 		return nil, 0, err