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>

mysql-test/main/explain_json.result

@@ -1544,7 +1544,9 @@ ANALYZE
       "buffer_size": "400",
       "join_type": "BKA",
       "mrr_type": "Rowid-ordered scan",
-      "r_filtered": 100
+      "r_loops": 10,
+      "r_filtered": 100,
+      "r_effective_rows": 1
     }
   }
 }