move UnitTestDefinition, implement missing resource_class methods

[MichelleArk]

resolves #9667

Problem

UnitTestDefinition, which the WritableManifest depends on, was defined within core

Solution

• Define UnitTestDefinition & and its upstream components in dbt/artifacts
• A bit of tidying around other resources -- implementing any missing resource_class methods and serializing them appropriately post-move.

Checklist

• I have read the contributing guide and understand what's expected of me
• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• This PR has no interface changes (e.g. macros, cli, logs, json artifacts, config files, adapter interface, etc) or this PR has already received feedback and approval from Product or DX
• This PR includes type annotations for new and modified functions

[github-actions]

Thank you for your pull request! We could not find a changelog entry for this change. For details on how to document a change, see the contributing guide.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.98%. Comparing base (7f9874d) to head (4846c8a).
Report is 7 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9651      +/-   ##
==========================================
- Coverage   88.00%   87.98%   -0.02%     
==========================================
  Files         176      178       +2     
  Lines       22311    22369      +58     
==========================================
+ Hits        19634    19682      +48     
- Misses       2677     2687      +10     

Flag	Coverage Δ
integration	85.43% <100.00%> (-0.16%)	⬇️
unit	62.15% <100.00%> (+0.05%)	⬆️

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