Add KernelAbstractions extension for matrix-free integral operators

[maltezfaria]

Implements a KernelAbstractions extension with a KernelMatrix structure to perform a matrix-free product. Tested on Metal for now. Missing:

• Modify API to allow for a compression = :kernelmatrix input (or something similar)
• Add the assemble_kernelmatrix(iop; backend = ...) to the page on compression methods on the docs

[codecov]

Codecov Report

❌ Patch coverage is 0% with 317 lines in your changes missing coverage. Please review.
✅ Project coverage is 0.00%. Comparing base (89bd588) to head (c9a56f8).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/kernels.jl	0.00%	166 Missing ⚠️
ext/IntiKernelMatrixExt.jl	0.00%	109 Missing ⚠️
src/api.jl	0.00%	16 Missing ⚠️
src/bdim.jl	0.00%	14 Missing ⚠️
src/utils.jl	0.00%	10 Missing ⚠️
src/nystrom.jl	0.00%	2 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (89bd588) and HEAD (c9a56f8). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (89bd588)	HEAD (c9a56f8)
	2	1

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #160       +/-   ##
==========================================
- Coverage   72.07%   0.00%   -72.08%     
==========================================
  Files          25      26        +1     
  Lines        4558    4752      +194     
==========================================
- Hits         3285       0     -3285     
- Misses       1273    4752     +3479     

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