MDEV-31577: Make ANALYZE FORMAT=JSON print innodb stats

ANALYZE FORMAT=JSON output now includes table.r_engine_stats which
has the engine statistics. Only non-zero members are printed.

Internally: EXPLAIN data structures Explain_table_acccess and
Explain_update now have handler* handler_for_stats pointer.
It is used to read statistics from handler_for_stats->handler_stats.

The following applies only to 10.9+, backport doesn't use it:

Explain data structures exist after the tables are closed. We avoid
walking invalid pointers using this:
- SQL layer calls Explain_query::notify_tables_are_closed() before
  closing tables.
- After that call, printing of JSON output is disabled. Non-JSON output
  can be printed but we don't access handler_for_stats when doing that.

mysql-test/main/explain_json_format_partitions.result

@@ -38,6 +38,7 @@ ANALYZE
       "r_rows": 10,
       "r_table_time_ms": "REPLACED",
       "r_other_time_ms": "REPLACED",
+      "r_engine_stats": REPLACED,
       "filtered": 100,
       "r_filtered": 30,
       "attached_condition": "t1.a in (2,3,4)"
@@ -60,6 +61,7 @@ ANALYZE
         "r_rows": 10,
         "r_filtered": 30,
         "r_total_time_ms": "REPLACED",
+        "r_engine_stats": REPLACED,
         "attached_condition": "t1.a in (2,3,4)"
       }
     }
@@ -80,6 +82,7 @@ ANALYZE
       "r_rows": 10,
       "r_filtered": 0,
       "r_total_time_ms": "REPLACED",
+      "r_engine_stats": REPLACED,
       "attached_condition": "t1.a in (20,30,40)"
     }
   }