feat: Do not require capnp to be installed to compile hugr-model

[aborgna-q]

Now we do the same as with the schema and extensions definition; provide a just command to update it manually and check on CI that things are up-to-date.

This lets us remove the build.rs from hugr-model, removing the need for users to have capnproto installed to compile our crates.

Closes #1667

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.75%. Comparing base (7312c62) to head (415975a).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1907      +/-   ##
==========================================
- Coverage   86.63%   81.75%   -4.88%     
==========================================
  Files         195      196       +1     
  Lines       35777    38962    +3185     
  Branches    32590    35775    +3185     
==========================================
+ Hits        30995    31853     +858     
- Misses       2987     5300    +2313     
- Partials     1795     1809      +14     

Flag	Coverage Δ
python	92.34% <ø> (ø)
rust	80.81% <ø> (-5.27%)	⬇️

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