add parametric model

[RaphaelS1]

@bblodfon if you could help tick off this list that would be very helpful to get this over the line

• Add tests
• Add test confirming lp <> risk order is same for all model forms
• Confirm same results from mlr3extralearners
• Add docs
• Add note that distr6 return is always TRUE if return type is continuous
• Reimplement in extralearners Move flexsurv and parametric to {survivalmodels} mlr-org/mlr3extralearners#41

[RaphaelS1]

@bblodfon tests now passing, please review when you have time

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 96.84211% with 6 lines in your changes are missing coverage. Please review.

Project coverage is 35.40%. Comparing base (9d59b0c) to head (feb9476).
Report is 8 commits behind head on main.

Files	Patch %	Lines
R/parametric.R	97.15%	5 Missing ⚠️
R/methods.R	75.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##              main      #45       +/-   ##
============================================
- Coverage   100.00%   35.40%   -64.60%     
============================================
  Files           16       18        +2     
  Lines          780      983      +203     
============================================
- Hits           780      348      -432     
- Misses           0      635      +635     

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