[Deadline Alerts] Dedupe Callbacks

### Description

I have collected user requests from various talks and meetups and now that Deadline Alerts are hitting GA, I am going to be dropping them into Issues so folks can pick them up.

User request: Dedupe Callbacks - "I don't want 100 emails saying this dag failed 100 times yesterday"

Assignment: Start a community discussion to determine if we want to implement this and, if so, drive implementation of some sort of throttling mechanism for Deadline callbacks.

### Use case/motivation

_No response_

### Related issues

_No response_

### Are you willing to submit a PR?

- [ ] Yes I am willing to submit a PR!

### Code of Conduct

- [x] I agree to follow this project's [Code of Conduct](https://github.com/apache/airflow/blob/main/CODE_OF_CONDUCT.md)

No files, tests, or entry points are named. Start by opening the requested community discussion to define whether callback throttling is wanted, what deduplication means for repeated DAG failures, and how completion should be evaluated; implementation can be scoped after that decision.

Written by the indexing model from the issue text.