Implement additional DeadlineReferences types

Implement at least `dag_run - queued-at` and `job - start-date`, maybe `serialized_dag - last_updated` as well?

These should be added in the form of DeadlineReference.DAGRUN_QUEUED_AT, etc.

These will include References for Assets, Task-level Deadlines, etc and is likely an entire project board on its own.

Start by locating DeadlineReference and the existing reference definitions and usages. Trace how current types represent resource timestamps, then determine the scope for dag_run queued-at and job start-date, including whether serialized_dag last_updated is included. Done means the agreed DeadlineReference constants and their corresponding behavior are implemented.

Written by the indexing model from the issue text.