Add a TransformFunctionDefinition dataclass

[frode-aarstad]

Issue Resolves #7610

Approach Short description of the approach

(Screenshot of new behavior in GUI if applicable)

• [x] PR title captures the intent of the changes, and is fitting for release notes.
• [x] Added appropriate release note label
• [ ] Commit history is consistent and clean, in line with the contribution guidelines.
• [ ] Make sure tests pass locally (after every commit!)

When applicable

• [ ] When there are user facing changes: Updated documentation
• [ ] New behavior or changes to existing untested code: Ensured that unit tests are added (See Ground Rules).
• [ ] Large PR: Prepare changes in small commits for more convenient review
• [ ] Bug fix: Add regression test for the bug
• [ ] Bug fix: Create Backport PR to latest release

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 98.27586% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 85.33%. Comparing base (338202e) to head (c56c148). Report is 1 commits behind head on main.

Files	Patch %	Lines
src/ert/config/gen_kw_config.py	97.61%	1 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7761      +/-   ##
==========================================
- Coverage   85.38%   85.33%   -0.06%     
==========================================
  Files         381      381              
  Lines       23593    23628      +35     
  Branches      881      877       -4     
==========================================
+ Hits        20144    20162      +18     
- Misses       3335     3353      +18     
+ Partials      114      113       -1     

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.