[Deadline Alerts] Implement a multiplier to the Average Runtime reference

### 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: Can we add some way to add a multiplier to the average rather than a fixed interval? If we don't know how long the average is, then we don't really know what "add five minutes" really means. A percentage would be more useful ### 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)