provide a post-build means to set messages sticky outside of config/environment file

this is more of a feature suggestion than a bug:

our team is looking to set an option for users to be able to toggle auto-dismiss/sticky messages. This enables us to meet accessibility standards for [WCAG-AA Success Criterion 4.1.3](https://www.w3.org/WAI/WCAG21/Understanding/timing-adjustable).

Currently, with every call we make to `ember-cli-flash`, we have to also pass `sticky: isStickySetOnForUser`. It would be great to have a place to set this based on user preferences, post-build rather than in the environment file.

thank you for your time <3
Seema