Add an API to cancel the migration

[kalroy]

User Story

As an Automate User, I should be able to cancel the migration on the preview screen.

Acceptance Criteria

• The API should only work if the migration is at "Create Preview" stage
• The API should clean up the migration folder, DB staging table and set the status to Cancelled migration

Implementation Details

• In this file, add this function.

Definition of Done

• All things specified in User Story Acceptance Criteria should be fulfilled.
• All Exceptions are Handled Properly
• Ensure logs have no unnecessary data.
• Test coverage for the new feature is done to at least 70%
• If needed raise Docs PR and tag documentation.
• Swagger Documentation updated
• Smoke Test done.
• Ensure Build and Integration Pipelines are Green.
• PR has 2 approvers.
• All Code Review Comments are Resolved.
• README doc should be updated, if needed.