feat: Userid mapping

[sattvikc]

Summary of change

Added user id mapping feature

Related issues

• Provide a user ID mapper to make migration easier supertokens-node#198

Test Plan

Documentation changes

(If relevant, please create a PR in our docs repo, or create a checklist here highlighting the necessary changes)

Checklist for important updates

• Changelog has been updated
• coreDriverInterfaceSupported.json file has been updated (if needed)
  • Along with the associated array in supertokens/constants.go
• [ ] frontendDriverInterfaceSupported.json file has been updated (if needed)
• Changes to the version if needed
  • In supertokens/constants.go > version variable
• Had installed and ran the pre-commit hook
• Issue this PR against the latest non released version branch.
  • To know which one it is, run find the latest released tag (git tag) in the format vX.Y.Z, and then find the latest branch (git branch --all) whose X.Y is greater than the latest released tag.
  • If no such branch exists, then create one from the latest released branch.

Remaining TODOs for this PR