MINOR: spoa-server: Allow registering external processes

Add struct for declaring an reistrering external processing resource.
diff --git a/contrib/spoa_server/spoa.h b/contrib/spoa_server/spoa.h
index 92c24ac..c8f9861 100644
--- a/contrib/spoa_server/spoa.h
+++ b/contrib/spoa_server/spoa.h
@@ -80,9 +80,22 @@
 	union spoe_value    u;     /* spoe data value */
 };
 
+struct spoe_kv {
+	struct chunk name;
+	struct spoe_data value;
+};
+
+struct ps {
+	struct ps *next;
+	char *ext;
+	int (*init_worker)(struct worker *w);
+};
+
 extern bool debug;
 extern pthread_key_t worker_id;
 
+void ps_register(struct ps *ps);
+
 #define LOG(fmt, args...) \
 	do { \
 		struct timeval now; \