refactor: introduce SessionCtx to hold non-state-machine state

[davidsteiner]

Group session context that doesn't change with the state machine into a single SessionCtx struct.

This will eventually allow us to pass the context borrowed from the session to session states so they can perform actions like manipulating the store and performing application callbacks.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.91%. Comparing base (4fd0ad7) to head (ab38d4c).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #321      +/-   ##
==========================================
+ Coverage   90.90%   90.91%   +0.01%     
==========================================
  Files          88       88              
  Lines        8781     8799      +18     
==========================================
+ Hits         7982     8000      +18     
  Misses        799      799              

Flag	Coverage Δ
core	90.78% <100.00%> (+0.01%)	⬆️

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