Compute capacity reservation for App Runner

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**Tell us about your request**
We're running App Runner in production and would like to guarantee compute capacity, and pay on-demand only when scaling out. This could be Saving Plans or other reservation.

This will negate the benefit of the idle compute saving, but it's better than unavailability in our case.

**Describe alternatives you've considered**
Migration to Fargate or EC2/Beanstalk is an option we're looking at, but not considering at the moment.