cmd/tailscale,ipn: add relay-server-port "tailscale set" flag and Prefs field (#15594)

This flag is currently no-op and hidden. The flag does round trip
through the related pref. Subsequent commits will tie them to
net/udprelay.Server. There is no corresponding "tailscale up" flag,
enabling/disabling of the relay server will only be supported via
"tailscale set".

This is a string flag in order to support disablement via empty string
as a port value of 0 means "enable the server and listen on a random
unused port". Disablement via empty string also follows existing flag
convention, e.g. advertise-routes.

Early internal discussions settled on "tailscale set --relay="<port>",
but the author felt this was too ambiguous around client vs server, and
may cause confusion in the future if we add related flags.

Updates tailscale/corp#27502

Signed-off-by: Jordan Whited <jordan@tailscale.com>

ipn/ipn_view.go

@@ -166,6 +166,10 @@ func (v PrefsView) NetfilterKind() string                 { return v.ж.Netfilte
 func (v PrefsView) DriveShares() views.SliceView[*drive.Share, drive.ShareView] {
 	return views.SliceOfViews[*drive.Share, drive.ShareView](v.ж.DriveShares)
 }
+func (v PrefsView) RelayServerPort() views.ValuePointer[int] {
+	return views.ValuePointerOf(v.ж.RelayServerPort)
+}
+
 func (v PrefsView) AllowSingleHosts() marshalAsTrueInJSON { return v.ж.AllowSingleHosts }
 func (v PrefsView) Persist() persist.PersistView          { return v.ж.Persist.View() }
 
@@ -200,6 +204,7 @@ var _PrefsViewNeedsRegeneration = Prefs(struct {
 	PostureChecking        bool
 	NetfilterKind          string
 	DriveShares            []*drive.Share
+	RelayServerPort        *int
 	AllowSingleHosts       marshalAsTrueInJSON
 	Persist                *persist.Persist
 }{})