fix: switch message field data structure to IndexMap from BTreeMap to maintain insertion order

[davidsteiner]

No description provided.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.48%. Comparing base (e8463e7) to head (fa5c00b).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #218   +/-   ##
=======================================
  Coverage   81.48%   81.48%           
=======================================
  Files          68       68           
  Lines        6140     6140           
=======================================
  Hits         5003     5003           
  Misses       1137     1137           

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