Changes

feat(pingcap/tidb): add new CI job for integration DDL tests next-gen (#3591)

- Introduce pipeline and pod configuration for
`pull_integration_ddl_test_next_gen`
- Register the job in `latest-presubmits-next-gen.yaml`
- Define stages for debugging, checkout, preparation, and testing

This commit enhances the CI/CD process for the TiDB project by adding a
dedicated job for integration DDL tests, ensuring better testing
coverage for next-gen features.


Close #3582

Signed-off-by: wuhuizuo <wuhuizuo@126.com>
 (commit: 521b286)