chore: Improve core v2 init API [MD-441]

[gt2345]

Ticket

MD-441

Description

Eliminate DefaultConfig and only use UnmanagedConfig in core_v2 init API

Test Plan

e2e_test pass

Checklist

• Changes have been manually QA'd
• New features have been approved by the corresponding PM
• User-facing API changes have the "User-facing API Change" label
• Release notes have been added as a separate file under docs/release-notes/
  See Release Note for details.
• Licenses have been included for new code which was copied and/or modified from any external code

[netlify]

✅ Deploy Preview for determined-ui canceled.

Name	Link
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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 49.80%. Comparing base (da9025b) to head (80e8f8a).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #9560   +/-   ##
=======================================
  Coverage   49.79%   49.80%           
=======================================
  Files        1247     1247           
  Lines      162235   162235           
  Branches     2888     2888           
=======================================
+ Hits        80793    80804   +11     
+ Misses      81270    81259   -11     
  Partials      172      172           

Flag	Coverage Δ
backend	43.89% <ø> (+0.02%)	⬆️
harness	63.74% <ø> (ø)
web	46.16% <ø> (ø)

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see 4 files with indirect coverage changes