[parser] ast: add explain format=json (#1055)
[parser] ast: add explain format=json (#1055)

Co-authored-by: kennytm <d10ee62456d8e2be8715605d23b69b7662d1915a@gmail.com>
pingcap/tidb,c4pt0r/tidb,kyle-wang/tidb,c4pt0r/tidb,c4pt0r/tidb,coocood/tidb,c4pt0r/tidb,pingcap/tidb,kyle-wang/tidb,pingcap/tidb,pingcap/tidb,kyle-wang/tidb,pingcap/tidb,kyle-wang/tidb,ngaut/tidb,pingcap/tidb,coocood/tidb,ngaut/tidb,ngaut/tidb,ngaut/tidb,kyle-wang/tidb,c4pt0r/tidb,c4pt0r/tidb,kyle-wang/tidb,ngaut/tidb,ngaut/tidb