add a script to migrate checkpoint table from preagg to raw.

[yu-shipit]

Pull Request checklist

• The commit(s) message(s) follows the contribution guidelines ?
• Tests for the changes have been added (for bug fixes / features) ?
• Docs have been added / updated (for bug fixes / features) ?

Current behavior : (link exiting issues here : https://help.github.com/articles/basic-writing-and-formatting-syntax/#referencing-issues-and-pull-requests)

New behavior :
Adding a tool to migrate checkpoint table from preagg to raw

BREAKING CHANGES

If this PR contains a breaking change, please describe the impact and migration
path for existing applications.
If not please remove this section.

Breaking changes may include:

• Any schema changes to any Cassandra tables
• The serialized format for Dataset and Column (see .toString methods)
• Over the wire formats for Akka messages / case classes
• Changes to the HTTP public API
• Changes to query parsing / PromQL parsing

Other information: