Consider using a Feature Toggle service for Mobile

[AntPolkanov]

(cc: @adamcogan, @matt-goldman, @tkapa, @zacharykeeping, @KristenHu)

Pain:
SSW Rewards functionality is growing and it would be nice to be able to toggle some features on and off without changes to the mobile app and the need to go through the Stores' approval process.

Solution:
Add Feature Toggle functionality.

Advantages:

• Better user experience - less app updates; not working functionality can be disabled
• Better development experience - separation of code deployment from feature releases; the team can small introduce incremental changes (smaller PRs) while keeping the app releasable if the new functionality can be placed behind a feature toggle
• Unlocks opportunities for A/B testing

Disadvantages:

• That's an additional step in the release process we should remember about
• We might need to pay for a Feature Toggle service
• Feature toggle config initialisation can be tricky

Feature Toggle services:

• Firebase
• ConfigCat
• Split