Migrate deadline references over to task sdk

[amoghrajesh]

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This is a follow up to: #61118, where deadline references needed to be better handled.

In this PR, I intend to complete the migration of deadline alert references from airflow-core to task-sdk. task SDK now contains lightweight reference definitions for DAG authoring, while core retains database aware evaluation logic.

Changes of Note

1. SDK: Added Lightweight ReferenceModels

Added reference classes with serialization only logic:

• BaseDeadlineReference - Base class with serialize_reference() / deserialize_reference()
• FixedDatetimeDeadline, DagRunLogicalDateDeadline, DagRunQueuedAtDeadline, AverageRuntimeDeadline
• Added _evaluate_with() stub (raises NotImplementedError) to support custom references

2. In airlfow-core, removed the ReferenceModels class but with __getattr__ redirect

3. SerializedReferenceModels stays in core with full database logic - no changes made there.

Sanity Run

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