rename fn_session_event_callback to plugin_event_callback and update API parameters

* plugin_event_callback delete 'struct stellar_packet *p'
    * plugin_event_callback rename 'uint16_t len' to 'size_t len'
    * plugin_event_callback rename 'void **pme' to 'void **ctx'

sdk/include/plugin.h

@@ -6,10 +6,13 @@ extern "C"
 {
 #endif
 
+#include <stddef.h>
+
 #include "session.h"
 
 typedef int plugin_init_callback(void);
 typedef void plugin_exit_callback(void);
+typedef void plugin_event_callback(const struct stellar_session *session, enum session_event_type event, const char *payload, size_t len, void **ctx);
 
 /******************************************************************************
  * Public API For Plugin
@@ -31,7 +34,7 @@ void pm_session_dettach_me(const struct stellar_session *session);
  *                                           /|\
  *                                            |
  *                         plugin cb2 run pm_session_take_over
- * 
+ *
  * A plugin(cb1/cb3/cb4) that is taken over, if the plugin was called before being taken over and has a registered SESSION_EVENT_CLOSING event,
  * it will be called again when the SESSION_EVENT_CLOSING event comes. Otherwise, the plugin will not be called.
  */