cmd/tailscale/cli: use optimistic concurrency control on SetServeConfig

This PR uses the etag/if-match pattern to ensure multiple calls
to SetServeConfig are synchronized. It currently errors out and
asks the user to retry but we can add occ retries as a follow up.

Updates #8489

Signed-off-by: Marwan Sulaiman <marwan@tailscale.com>

ipn/ipn_view.go

@@ -182,6 +182,7 @@ func (v ServeConfigView) Foreground() views.MapFn[string, *ServeConfig, ServeCon
 		return t.View()
 	})
 }
+func (v ServeConfigView) ETag() string { return v.ж.ETag }
 
 // A compilation failure here means this code must be regenerated, with the command at the top of this file.
 var _ServeConfigViewNeedsRegeneration = ServeConfig(struct {
@@ -189,6 +190,7 @@ var _ServeConfigViewNeedsRegeneration = ServeConfig(struct {
 	Web         map[HostPort]*WebServerConfig
 	AllowFunnel map[HostPort]bool
 	Foreground  map[string]*ServeConfig
+	ETag        string
 }{})
 
 // View returns a readonly view of TCPPortHandler.