refactor: update codebase to use Pydantic v2

[lsetiawan]

Overview

This PR refactor the pydantic models to use Pydantic version 2 and its best practices. Additionally, the version for autodoc_pydantic is now bumped to 2 as well.

Closes #85

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (533b20a) 43.39% compared to head (1ee890b) 43.39%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #133   +/-   ##
=======================================
  Coverage   43.39%   43.39%           
=======================================
  Files          15       15           
  Lines         613      613           
=======================================
  Hits          266      266           
  Misses        347      347           

Files Changed	Coverage Δ
src/gnatss/loaders.py	22.80% <100.00%> (ø)

... and 1 file with indirect coverage changes

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[aniketfadia96]

I haven't used Pydantic before so I don't have major comments. Added a minor comment, but overall LGTM.