Changes

prow: make pull_integration_ddl_test optional and manual-trigger (#4123)

### What changed
- Make `pingcap/tidb/pull_integration_ddl_test` non-blocking and
manual-trigger only at the prow presubmit level by setting:
  - `always_run: false`
  - `optional: true`

### How to trigger
- Comment on PR:
  - `/test pull-integration-ddl-test`

### Scope
- `ci/prow-jobs/pingcap/tidb/latest-presubmits.yaml` only.
 (commit: 42c19ef)