Storage strategies

[slovnicki]

This is a work in progress proposal and will be updated with implementation details

For more details about where this is coming from, visit v2 Roadmap Discussion or v2 Roadmap Wiki.

Motivation

This is inspired by bottom_navigation_riverpod example by @vduseev where SharedPreferences is used to store the navigation state of each Beamer which then preserves their states if we open the app in a different tab, rebuild the the app by typing the URL, login/logout from the app, etc. We would like to add the storage capability into Beamer directly so we can turn it on/off and access at any time. Developer should also be able to choose their own storage strategy which just needs to implement "BeamerStorage" interface.