Implement map key exists in mgp (#1336)

include/mg_procedure.h

@@ -482,6 +482,9 @@ enum mgp_error mgp_map_size(struct mgp_map *map, size_t *result);
 /// Result is NULL if no mapping exists.
 enum mgp_error mgp_map_at(struct mgp_map *map, const char *key, struct mgp_value **result);
 
+/// Returns true if key in map.
+enum mgp_error mgp_key_exists(struct mgp_map *map, const char *key, int *result);
+
 /// An item in the mgp_map.
 struct mgp_map_item;