IR: Update to Pydantic >2.0 compatibility

[mlange05]

Note:` For now, just for testing...

This PR brings a few enabling changes to the internal IR node definitions that allow us to lift the version restriction on pydantic. We needed this previously, as we were relying on some (broken) default behaviour that was fixed upstream in pydantic version 2. By changing to the correct type annotations, in particular for Optional arguments, we can now lift this restriction.

In addition, I also added a small set of rudimentary checks for the constructor and object behaviour (immutability) for three most common/interesting node types. I plan to expand on this somewhat in a follow-up PR that will require pydantic features from version 2.x.

In some more detail:

• Use Optional throughout node type annotations to allow default values of None
• Add a rudimentary test for basic constructor behaviour and "frozeness" of our IR nodes, for Assignment, Loop and Conditional
• Lift version restriction on pydantic and add a few small fixes to deal with the fallout

Note: This is the first of two PRs and paves the way for more cool stuff to come (as discussed offline).

[github-actions]

Documentation for this branch can be viewed at https://sites.ecmwf.int/docs/loki/349/index.html

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.30%. Comparing base (78e7605) to head (07d89ec).
Report is 8 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #349      +/-   ##
==========================================
+ Coverage   95.29%   95.30%   +0.01%     
==========================================
  Files         170      171       +1     
  Lines       36414    36490      +76     
==========================================
+ Hits        34701    34778      +77     
+ Misses       1713     1712       -1     

Flag	Coverage Δ
lint_rules	96.39% <ø> (ø)
loki	95.28% <100.00%> (+0.01%)	⬆️

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[reuterbal]

This looks great, many thanks for grinding this through!