Refactor Asset Decorator to Use SdkDAG for Improved Modularity (#46927)

[shashbha14]

This PR introduces SdkDAG inside the SDK to replace the direct dependency on airflow.models.dag within the asset decorator. This improves modularity and aligns with the project’s architectural principles.

Changes:
Implemented SdkDAG within airflow.sdk.dag.
Updated asset decorators (@asset and @asset.multi) to use SdkDAG instead of airflow.models.dag.
Ensured compatibility with existing asset definitions.
Why This Change?
Addresses Issue #46927.
Provides a structured way to define DAGs within the SDK.
Improves maintainability by reducing dependencies on core Airflow modules.
Next Steps:
Run full test suite to validate the changes.
Gather feedback from maintainers for further refinements.

[boring-cyborg]

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