feat: add new ConstantOrCache context

[gdalle]

• Add new context type ConstantOrCache which is passed as-is to the backend, without element type modification (unlike Cache). This is meant to be used in conjunction with PreallocationTools.jl.
• Add corresponding tests to DIT and to a few backends.

[codecov]

Codecov Report

Attention: Patch coverage is 99.50980% with 1 line in your changes missing coverage. Please review.

Project coverage is 97.87%. Comparing base (7d252ad) to head (cf73f95).

Files with missing lines	Patch %	Lines
DifferentiationInterfaceTest/src/utils.jl	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #749      +/-   ##
==========================================
+ Coverage   97.85%   97.87%   +0.01%     
==========================================
  Files         128      128              
  Lines        7383     7423      +40     
==========================================
+ Hits         7225     7265      +40     
  Misses        158      158              

Flag	Coverage Δ
DI	98.94% <100.00%> (+<0.01%)	⬆️
DIT	95.20% <98.03%> (+0.08%)	⬆️

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