[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)