dialects: (snitch_stream) add stride pattern attribute

[superlopuh]

PR 2/4 of replacing the snitch_stream.stride_pattern op with an attribute

This introduces the new attribute, currently unused

[codecov]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (b18b0df) 90.17% compared to head (df804d9) 90.17%.

Files	Patch %	Lines
xdsl/dialects/snitch_stream.py	89.65%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@                                  Coverage Diff                                   @@
##           sasha/snitch_stream/stride-pattern-lowering-helper    #2141      +/-   ##
======================================================================================
- Coverage                                               90.17%   90.17%   -0.01%     
======================================================================================
  Files                                                     305      305              
  Lines                                                   37043    37072      +29     
  Branches                                                 5515     5523       +8     
======================================================================================
+ Hits                                                    33404    33430      +26     
- Misses                                                   2860     2862       +2     
- Partials                                                  779      780       +1     

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