core: Add optional attributes to declarative assembly format

[lucjaulmes]

I was going to suggest the missing optional $inner_sym attr on #2404 but realised optional attributes don’t exist yet in the declarative format. It seemed a rather trivial extension so here’s my shot at it, let me know if something misses/feels wrong.

[codecov]

Codecov Report

Attention: Patch coverage is 93.75000% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 89.68%. Comparing base (d1f2cad) to head (48f5cd8).

Files	Patch %	Lines
xdsl/irdl/declarative_assembly_format.py	83.33%	0 Missing and 3 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2407   +/-   ##
=======================================
  Coverage   89.68%   89.68%           
=======================================
  Files         346      346           
  Lines       41418    41465   +47     
  Branches     6175     6183    +8     
=======================================
+ Hits        37144    37188   +44     
  Misses       3372     3372           
- Partials      902      905    +3     

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[lucjaulmes]

I’ve added optional property too to fix coverage issues and for completeness.