MINOR: stream/applet: add use-service action

This new target can be called from the frontend or the backend. It
is evaluated just before the backend choice and just before the server
choice. So, the input stream or HTTP request can be forwarded to a
server or to an internal service.
diff --git a/include/types/action.h b/include/types/action.h
index ca0163f..b97f9bf 100644
--- a/include/types/action.h
+++ b/include/types/action.h
@@ -24,6 +24,7 @@
 
 #include <common/regex.h>
 
+#include <types/applet.h>
 #include <types/stick_table.h>
 
 enum act_from {
@@ -101,6 +102,7 @@
 	enum act_return (*action_ptr)(struct act_rule *rule, struct proxy *px,  /* ptr to custom action */
 	                              struct session *sess, struct stream *s, int flags);
 	struct action_kw *kw;
+	struct applet applet;                  /* used for the applet registration. */
 	union {
 		struct {
 			char *realm;