Add ability for multipart data handler to provide pushback

It can specify how much data was actually processed and the rest will be re-delivered on next poll.

CL: mg: Add ability for multipart data handler to provide pushback

PUBLISHED_FROM=e0168c5064c3a32921c9209bc09f4da2079cd616

docs/c-api/mg_http.h/struct_mg_http_multipart_part.md

@@ -9,6 +9,14 @@ signature: |
     struct mg_str data;
     int status; /* <0 on error */
     void *user_data;
+    /*
+     * User handler can indicate how much of the data was consumed
+     * by setting this variable. By default, it is assumed that all
+     * data has been consumed by the handler.
+     * If not all data was consumed, user's handler will be invoked again later
+     * with the remainder.
+     */
+    size_t num_data_consumed;
   };
 ---