Start implementing Cache contexts

[gdalle]

Versions

• Bump DI to v0.6.15
• Bump DIT to v0.8.4

DI source

• Implement Cache <: Context the same way as Constant

DI docs

• Explain the difference between Cache and Constant
• Specify which context types are implemented for which backends

DIT source

• Add a new scenario modified cachify which takes an additional context y_cache where the result is stored before being returned
• When checking scenario equality, ignore Cache contexts
• Introduce a new abstract type FunctionModifier for the scenario fiddlers

DI and DIT tests

• Test cachified scenarios with finite difference backends

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 94.44444% with 3 lines in your changes missing coverage. Please review.

Project coverage is 97.57%. Comparing base (d4b17c1) to head (15fec84).

Files with missing lines	Patch %	Lines
DifferentiationInterface/src/utils/context.jl	60.00%	2 Missing ⚠️
...Ext/DifferentiationInterfaceTestStaticArraysExt.jl	93.75%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #587      +/-   ##
==========================================
- Coverage   97.58%   97.57%   -0.01%     
==========================================
  Files         110      110              
  Lines        5208     5243      +35     
==========================================
+ Hits         5082     5116      +34     
- Misses        126      127       +1     

Flag	Coverage Δ
DI	98.59% <60.00%> (-0.06%)	⬇️
DIT	95.70% <97.95%> (+0.12%)	⬆️

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