add a new explain option including_query for include query string inside the json plan output

for json format, add a new option to let the explain json output also
include the actual query string.

it can make json usage more convenient.
Now you only need to grab the json output, no need to
collect another explain statement and extract the actual query from
the explain statement.

including_query name is so far what i can come up with, if people have
better ideas, then we can change.

example:
explain (analyze,including_query on, format json) select 1;
QUERY PLAN
-------------------------------------
[ +
{"Query": "select 1"}, +
{ +
"Plan": { +
"Node Type": "Result", +
"Parallel Aware": false, +
"Async Capable": false, +
"Startup Cost": 0.00, +
"Total Cost": 0.01, +
"Plan Rows": 1, +
"Plan Width": 4, +
"Actual Startup Time": 0.001,+
"Actual Total Time": 0.001, +
"Actual Rows": 1, +
"Actual Loops": 1 +
}, +
"Planning Time": 0.119, +
"Triggers": [ +
], +
"Execution Time": 0.033 +
} +
]
(1 row)