Added columns as case values

[ilongin]

Up until now we only allowed python primitives as case values like str, int, float ...
With this PR we will allow columns as well (note that this also adds ability for columns as outcomes in ifelse as it uses case under the hood ), e.g

# case example
res = dc.mutate(test=case((C("num") < 3, C("val")), else_="default"))

# ifelse example
res = dc.mutate(test=ifelse(C("num") < 3, C("val"), "default"))

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.65%. Comparing base (143b4be) to head (fbf1ee1).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #872      +/-   ##
==========================================
- Coverage   87.65%   87.65%   -0.01%     
==========================================
  Files         128      128              
  Lines       11457    11461       +4     
  Branches     1544     1546       +2     
==========================================
+ Hits        10043    10046       +3     
- Misses       1026     1027       +1     
  Partials      388      388              

Flag	Coverage Δ
datachain	87.57% <100.00%> (-0.01%)	⬇️

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[dreadatour]

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