MDEV-36483: store ddls in the optimizer trace

This feature stores the ddls of the tables/views that are used in
a query, to the optimizer trace. It is currently controlled by a
system variable store_ddls_in_optimizer_trace, and is not enabled
by default. All the ddls will be stored in a single json array, with each
element having table/view name, and the associated create definition
of the table/view.

The approach taken is to read global query_tables from the thd->lex,
and read them in reverse. Create a record with table_name, ddl of
the table and add the table_name to the hash,
along with dumping the information to the trace.
dbName_plus_tableName is used as a key,
and the duplicate entries are not added to the hash.

The main suite tests are also run with the feature enabled, and they
all succeed.

Author: bsrikanth-mariadb

libmysqld/CMakeLists.txt

@@ -161,6 +161,7 @@ SET(SQL_EMBEDDED_SOURCES emb_qcache.cc libmysqld.c lib_sql.cc
            ../sql/opt_hints_parser.cc ../sql/opt_hints_parser.h
            ../sql/scan_char.h
            ../sql/opt_hints.cc ../sql/opt_hints.h
+	   ../sql/opt_trace_ddl_info.cc ../sql/opt_trace_ddl_info.h
            ${GEN_SOURCES}
            ${MYSYS_LIBWRAP_SOURCE}
 )

mysql-test/main/mysqld--help.result

@@ -858,6 +858,9 @@ The following specify which files/extra groups are read (specified before remain
  heuristic, thus perform exhaustive search: 1 - prune
  plans based on cost and number of retrieved rows eq_ref:
  2 - prune also if we find an eq_ref chain
+ --optimizer-record-context 
+ Controls storing of optmizer context of all the tables
+ that are referenced in a query
  --optimizer-row-copy-cost=# 
  Cost of copying a row from the engine or the join cache
  to the SQL layer
@@ -1891,6 +1894,7 @@ optimizer-key-next-find-cost 0.082347
 optimizer-max-sel-arg-weight 32000
 optimizer-max-sel-args 16000
 optimizer-prune-level 2
+optimizer-record-context FALSE
 optimizer-row-copy-cost 0.060866
 optimizer-row-lookup-cost 0.130839
 optimizer-row-next-find-cost 0.045916