Replace closures with callable structs in second order

[gdalle]

DI source

• Replace inner closures with callable structs for hvp and second_derivative (idea taken from Self-preparing inner closures for second order #291)
• Replace v with dx in hvp source code and docstrings, to be coherent with pushforward and pullback

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 97.29730% with 2 lines in your changes missing coverage. Please review.

Project coverage is 96.04%. Comparing base (e93d16a) to head (86af6ab).

Files	Patch %	Lines
DifferentiationInterface/src/second_order/hvp.jl	96.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #315      +/-   ##
==========================================
- Coverage   96.08%   96.04%   -0.05%     
==========================================
  Files          95       95              
  Lines        4398     4403       +5     
==========================================
+ Hits         4226     4229       +3     
- Misses        172      174       +2     

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