planner: Explain Explore returns a new column to help user create bindings more easily

## Enhancement
Now `Explain Explore` returns a SQL like `CREATE GLOBAL BINDING FROM HISTORY USING PLAN DIGEST 'fb7ba1191ca76f8d9a73c7180d7ba3cbb4043dec48ceb45752d0796006a190c7'` to help users create such a binding.
But this `USING PLAN DIGEST` command requires this plan to be stored in `mysql.statement_summary`, but on the cloud, `mysql.statement_summary` might not be available.
But if we return the whole original query like `CREATE GLOBAL BINDING USING SELECT /*+ */ * FROM ...`, the result could be too long and contain some invisible characters, which prevent the user from copying and pasting the command to create such a binding.

To solve this, maybe we could return a new column like `CREATE GLOBAL BINDING USING 'a3F...xtP'`, where `a3F...xtP` is base64 encoded and compressed from the original query, which is used to make the result shorter and remove all invisible characters.