feat: add getter and setter for initial values

[sebffischer]

• This can be useful if one (e.g. when tuning) wants to run the learner with the default configuration without reconstructing the learner.
• It can be checked whether a learner has defaults set, by doing is.null(param_set$initial_values).
• The param_set$values = list(...) in the learner / pipeops initialize function can simply be replaced with param_set$set_initial_values(...)

[codecov-commenter]

Codecov Report

Merging #374 (fc6d535) into main (ad3dd06) will decrease coverage by 0.06%. The diff coverage is 90.47%.

:exclamation: Current head fc6d535 differs from pull request most recent head 60b8ad3. Consider uploading reports for the commit 60b8ad3 to get more accurate results

@@            Coverage Diff             @@
##             main     #374      +/-   ##
==========================================
- Coverage   93.48%   93.42%   -0.07%     
==========================================
  Files          26       26              
  Lines        1044     1064      +20     
==========================================
+ Hits          976      994      +18     
- Misses         68       70       +2     

Impacted Files	Coverage Δ
R/ParamSetCollection.R	94.56% <81.81%> (-1.78%)	:arrow_down:
R/ParamSet.R	95.88% <100.00%> (+0.13%)	:arrow_up:

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