feat: Add string support to IOKey values

[mehmetozsoy-synnada]

Description

Closes #10.

What is Changed

• Two tests added for IOKey string value handling
• Added str to corresponding fields

Checklist:

• Tests that cover the code added.
• Corresponding changes documented.
• All tests passed.
• The code linted and styled (pre-commit run --all-files has passed).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.69%. Comparing base (725b745) to head (727e652).
Report is 1 commits behind head on main.

@@           Coverage Diff           @@
##             main      #11   +/-   ##
=======================================
  Coverage   85.69%   85.69%           
=======================================
  Files          61       61           
  Lines       15224    15224           
=======================================
  Hits        13046    13046           
  Misses       2178     2178           

Files with missing lines	Coverage Δ
mithril/framework/codegen/python_gen.py	95.67% <ø> (ø)
mithril/framework/common.py	95.76% <ø> (ø)
mithril/framework/constraints.py	94.26% <100.00%> (ø)
mithril/framework/logical/base.py	87.44% <100.00%> (ø)
mithril/framework/logical/model.py	96.25% <100.00%> (ø)
mithril/framework/physical/data_store.py	95.56% <100.00%> (ø)
mithril/framework/physical/flat_graph.py	95.58% <ø> (ø)