Add type annotations on f in DI

[gdalle]

Core

• Add f::F and where {F} to the signature of every single operator, in order to force specialization

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 97.36842% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 96.13%. Comparing base (cc73842) to head (9167758).
Report is 2 commits behind head on main.

Files	Patch %	Lines
...fferentiationInterface/src/first_order/pullback.jl	85.71%	1 Missing ⚠️
...rentiationInterface/src/first_order/pushforward.jl	83.33%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #231   +/-   ##
=======================================
  Coverage   96.13%   96.13%           
=======================================
  Files          74       74           
  Lines        3911     3911           
=======================================
  Hits         3760     3760           
  Misses        151      151           

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