feat: Add python definitions for the TKET metadata keys

[aborgna-q]

We should also add a metadata for inline hints either here or in #1532, whichever gets merged second.

[codecov]

Codecov Report

❌ Patch coverage is 66.03774% with 18 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.77%. Comparing base (9a25ec8) to head (31b6b81).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
tket-py/tket/metadata.py	65.38%	18 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1535      +/-   ##
==========================================
- Coverage   83.79%   83.77%   -0.02%     
==========================================
  Files         188      189       +1     
  Lines       29032    29084      +52     
  Branches    27928    27928              
==========================================
+ Hits        24326    24364      +38     
- Misses       3543     3558      +15     
+ Partials     1163     1162       -1     

Flag	Coverage Δ
python	91.06% <66.03%> (-1.27%)	⬇️
qis-compiler	91.66% <ø> (ø)
rust	83.46% <ø> (+0.01%)	⬆️

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