MDEV-30806: ANALYZE FORMAT=JSON: better support for BNL and BNL-H joins

In block-nl-join, add:

- r_loops - this shows how many incoming record combinations this
  query plan node had.

- r_effective_rows - this shows the average number of matching rows
  that this table had for each incoming record combination. This is
  comparable with r_rows in non-blocked access methods.
  For BNL-joins, it is always equal to
   $.table.r_rows * $.table.r_filtered
  For BNL-H joins the value cannot be computed from other values

Reviewed by: Monty <monty@mariadb.org>

sql/sql_select.h

@@ -309,6 +309,7 @@ typedef struct st_join_table {
   Table_access_tracker *tracker;
 
   Table_access_tracker *jbuf_tracker;
+  Counter_tracker  *jbuf_loops_tracker;
   /* 
     Bitmap of TAB_INFO_* bits that encodes special line for EXPLAIN 'Extra'
     column, or 0 if there is no info.