Add migrate endpoint for push repositories

[gerrod3]

Summary

• Add a migrate action on push repositories that dispatches a task to convert ContainerPushRepository into ContainerRepository
• Support optional copy_versions to preserve full repository version history (defaults to copying only the latest version)
• Reassociate distributions, return the created repository in the task result, and add functional test coverage

Test plan

• Run pytest pulp_container/tests/functional/api/test_migrate_push_repository.py
• Verify POST /pulp/api/v3/repositories/container/container-push/{pk}/migrate/ returns 202 and completes successfully
• Confirm migrated repository appears under container repositories with preserved content and distribution link
• Test copy_versions: true when multiple repository versions exist

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