feat: Add ExtensionOp helpers

[aborgna-q]

• We had OpType::cast but were missing it in ExtensionOp itself.
• Adds a ExtensionOp::extension_id call

[codecov]

Codecov Report

Attention: Patch coverage is 57.14286% with 3 lines in your changes missing coverage. Please review.

Project coverage is 82.90%. Comparing base (7626797) to head (d4cbe79).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
hugr-core/src/ops/custom.rs	50.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2072      +/-   ##
==========================================
- Coverage   82.91%   82.90%   -0.01%     
==========================================
  Files         217      217              
  Lines       41529    41534       +5     
  Branches    37707    37712       +5     
==========================================
+ Hits        34433    34435       +2     
- Misses       5292     5295       +3     
  Partials     1804     1804              

Flag	Coverage Δ
python	85.40% <ø> (ø)
rust	82.65% <57.14%> (-0.01%)	⬇️

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[doug-q]

Wonderful, thank you!