Add OpenCL extension and buildkite CI

[VarLad]

Most of the code is taken from #43

Closes JuliaGPU/OpenCL.jl#280

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 31 lines in your changes missing coverage. Please review.

Project coverage is 53.18%. Comparing base (e60c518) to head (864cce1).
Report is 14 commits behind head on main.

Files with missing lines	Patch %	Lines
ext/AtomixOpenCLExt.jl	0.00%	31 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main      #51       +/-   ##
===========================================
- Coverage   92.95%   53.18%   -39.78%     
===========================================
  Files           6       10        +4     
  Lines         142      267      +125     
===========================================
+ Hits          132      142       +10     
- Misses         10      125      +115     

Flag	Coverage Δ
Pkg.test	53.18% <0.00%> (-39.78%)	⬇️

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[maleadt]

Tests are pretty lightweight, but other extensions do the same, so it's fine for now I guess.