Implement contexts for more backends

[gdalle]

DI extensions

• FiniteDifferences: Allow contexts.
• ForwardDiff: Extend contexts beyond pushforward using closures.
• Tracker: Allow contexts.
• Zygote: Extend contexts beyond pullback using closures.

DI tests

• Add tests with constants for FiniteDifferences and Tracker.

DIT source

• Fix some default scenarios num_to_num_vec so that they preserve array type (and thus tracking).

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.32%. Comparing base (472c027) to head (3194c2b).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #492      +/-   ##
==========================================
+ Coverage   98.30%   98.32%   +0.02%     
==========================================
  Files         106      106              
  Lines        4309     4364      +55     
==========================================
+ Hits         4236     4291      +55     
  Misses         73       73              

Flag	Coverage Δ
DI	98.34% <100.00%> (+0.03%)	⬆️
DIT	98.28% <100.00%> (ø)

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