dialects: (onnx) add onnx.EntryPoint

[kayode-gif]

resolves #2067

[kayode-gif]

note btw: i didn't include any verification function, as i didn't believe there was anything further to verify (in the onnx-mlir there is not anything too verify also)

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (87c26a7) 90.08% compared to head (36e4d4f) 90.08%.

Files	Patch %	Lines
xdsl/dialects/onnx.py	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2078      +/-   ##
==========================================
- Coverage   90.08%   90.08%   -0.01%     
==========================================
  Files         293      293              
  Lines       36570    36576       +6     
  Branches     5433     5434       +1     
==========================================
+ Hits        32944    32949       +5     
- Misses       2855     2856       +1     
  Partials      771      771              

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