Add dedupe logic for Deadline Alerts (#63856)

[Lavanya14-Y]

Fixes #63856

• Adds logic to prevent duplicate Deadline Alert emails
• Example output:

Sending email for dag_1 to abc@example.com
Skipping email for dag_1
Sending email for dag_2 to xyz@example.com

Implement deduplication for email alerts based on DAG ID.

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[Lavanya14-Y]

Fixes #63856
Please review this PR. I have added deduplication logic for Deadline Alerts.

[Srabasti]

Looks like Apache License is missing in file header @Lavanya14-Y, as a result static checks are failing.
Try running prek in your local branch and then commit to Airflow github repo.
Remember to rebase to ensure latest fixes are in!

[Srabasti]

The tests should be in a different file I believe, as unit tests, and not in the source code, like other functionality. Thoughts on this?