[metal] Dynamic Threadgroup memory

[christiangnrd]

Goes with JuliaGPU/Metal.jl#750.

This approach is only supported in macOS 15+, but it felt like the best approach for me to start with to learn about how this stuff all works.

Not yet functional, I'm not sure how to get the type with the opaque pointers (or if that's even how I would need to do it)

[codecov]

Codecov Report

❌ Patch coverage is 29.72973% with 26 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.01%. Comparing base (8adcb92) to head (3def2c5).

Files with missing lines	Patch %	Lines
src/metal.jl	29.72%	26 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #768      +/-   ##
==========================================
+ Coverage   77.95%   78.01%   +0.06%     
==========================================
  Files          25       25              
  Lines        4545     4554       +9     
==========================================
+ Hits         3543     3553      +10     
+ Misses       1002     1001       -1     

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