[Force upgrade] Fetch app config from github pages endpoint and marshal it accordingly

[owencraston]

Description

• fetch the AppConfig from the Github pages endpoint and marshal it to a useable typescript interface
• this involves
  • defining api fetching best practices for typescript
  • deciding if we need a library for this, if so what library

Builds off of

• https://github.com/MetaMask/metamask-mobile/pull/4483/files
• https://github.com/MetaMask/metamask-mobile/issues/4810

#Situation

• not user facing

Acceptance Criteria

• when the app loads, it fetches the object and converts it into a useable interface within the app

Process (adapt per story)

• [ ] User research / data
• [ ] Ideation / Kickoff in cross-functional team
• [ ] Pattern research in mobile (crypto / non-crypto space)
• [ ] Proof of concept to check technical feasibility (if applicable)
• [ ] Design reviews with PM, Engineer(s)
• [ ] User testing
• [ ] Design - Dev handoff
• [x] Implementation
• [x] Code review
• [ ] Stakeholder review on latest build on iOS and Android
• [ ] QA - unit test, e2e tests, regression testing