Add constant contexts for Enzyme

[gdalle]

DI extensions

• Enzyme: Support Constant contexts by translating them into Enzyme.Const.
• Zygote: Type-annotate Vararg{Context,C} to specialize on the number of arguments.

DI docs

• Specify that contexts now work with ForwardDiff, Zygote and Enzyme.

DI tests

• Enzyme: Add tests with constantified scenarios

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.87%. Comparing base (1369010) to head (4ef5b09).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #489      +/-   ##
==========================================
+ Coverage   97.84%   97.87%   +0.02%     
==========================================
  Files         107      107              
  Lines        4311     4322      +11     
==========================================
+ Hits         4218     4230      +12     
+ Misses         93       92       -1     

Flag	Coverage Δ
DI	98.31% <100.00%> (+<0.01%)	⬆️
DIT	96.95% <ø> (+0.07%)	⬆️

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