fix: handle function contexts differently from constant contexts

[gdalle]

DI source

• Define FunctionContext and BackendContext as private subtypes of Context, useful in second-order operators when passing stuff to the shuffled first-order operator
• Add shortcut ConstantOrFunctionOrBackend = Union{Constant,FunctionContext,BackendContext}

DI extensions

• Replace DI.Constant with DI.ConstantOrFunctionOrBackend in most places
• ForwardDiff: define PrepContext internally for second-order preparation with the right Dual type
• Enzyme: handle FunctionContext differently by annotating it as Duplicated if necessary. Function annotation doesn't work for second-order Enzyme #592 is still not fixed but at least it should error in the second order case.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.60%. Comparing base (6806fef) to head (2186d82).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #660      +/-   ##
==========================================
+ Coverage   97.58%   97.60%   +0.01%     
==========================================
  Files         112      112              
  Lines        5551     5592      +41     
==========================================
+ Hits         5417     5458      +41     
  Misses        134      134              

Flag	Coverage Δ
DI	98.75% <100.00%> (+0.01%)	⬆️
DIT	95.36% <ø> (ø)

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