feat: backend endpoints for instance creation and dataelement operations

[Jondyr]

Description

• Refactored preview instances to be mocked closer to how app-lib-core handles instances.
• Restructured controllers on the backend to more closely resemble app-lib-core structure. This should make it easier to reference app-lib-core when editing/updating the mock in the future.
• Moved preview support for V3 apps to separate controllers in a new folder to separate the legacy code. This should make maintenance/refactoring a bit easier.

This change allows the preview to use multiple datamodels without mocking every datamodel explicitly.

Related Issue(s)

• Preview Fails When Linking Component to Non-Default Data Model #14104

Verification

• Your code builds clean without any errors or warnings
• Manual testing done (required)
• Relevant automated test added (if you find this hard, leave it and we'll help out)

Documentation

• User documentation is updated with a separate linked PR in altinn-studio-docs. (if applicable)

[codecov]

Codecov Report

Attention: Patch coverage is 96.87500% with 1 line in your changes missing coverage. Please review.

Project coverage is 95.53%. Comparing base (9f43969) to head (155a940).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
...kages/ux-editor/src/components/Preview/Preview.tsx	90.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #14154   +/-   ##
=======================================
  Coverage   95.52%   95.53%           
=======================================
  Files        1817     1815    -2     
  Lines       23700    23694    -6     
  Branches     2749     2752    +3     
=======================================
- Hits        22640    22636    -4     
+ Misses        807      805    -2     
  Partials      253      253           

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[lassopicasso]

When this is merged, move #13797 from blocked to test. :)