Simplifying flattenObject(), adding support for non-string types

[olemartinorg]

Description

Lots of stuff will fail here, but this targets next (aka v4), so it's fine to break things for now (I think). This is the first step towards supporting non-string types. Next steps would be to make sure we look up the bound data type in the JsonSchema to know how we should store the data for a given component.

Related Issue(s)

• Considerations for v4 #339
• Analysis: get rid of stringified form data in frontend? Produced JSON invalid against JSON-schema. #205

Verification

• Manual testing
  • I have tested these changes manually
  • Creator of the original issue (or service owner) has been contacted for manual testing
  • No testing done/necessary
• Automated tests
  • Unit test(s) have been added
  • Cypress E2E test(s) have been added
  • No automatic tests are needed here
  • I want someone to help me make some tests
• User documentation @ altinn-studio-docs
  • Has been updated
  • No changes/updates needed
• Changes/additions to component properties
  • Changes are reflected in both layout/index.ts and layout.schema.v1.json, and these are all backwards-compatible
  • No changes made
• Support in Altinn Studio
  • Issue(s) created for support in Studio
  • This change/feature does not require any changes to Altinn Studio
• Sprint board
  • The original issue (or this PR itself) has been added to the Team Apps project and to the current sprint board

[olemartinorg]

Closing this one. It is very outdated now, and I'm working on supporting non-stringy data in data models when adding support for the new PATCH endpoint, so this direction is not the future I'm rooting for anyway.