Support json_unquote function

[yibin87]

Enhancement

json_unquote only accepts string/json data type. When json type parameter is met, it will be cast as string before being fed into json_unquote like this:
json_unquote(cast(json as string))
TiFlash has already support basic functionality of json_unquote and cast_json_as_string in #6377

Two main work remain todo:

1. Check if string is valid when quoted.
   tidb implementation
2. cast_json_as_string: already support cast json as unrestricted string. However, the output string data type can be restricted, like char(20).
   tidb implementation