Mark some of the indices in olm/megolm messages as required

[poljar]

This ensures that the fields won't be omitted if they contain the default value 0.

This removes the need for our custom encode_manual methods while retaining libolm compatibility.

Took a while before coming back to this.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.63%. Comparing base (cfc4eee) to head (562e055).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #405      +/-   ##
==========================================
- Coverage   91.72%   91.63%   -0.09%     
==========================================
  Files          40       40              
  Lines        5533     5475      -58     
  Branches     5533     5475      -58     
==========================================
- Hits         5075     5017      -58     
  Misses        287      287              
  Partials      171      171              

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[codspeed-hq]

Merging this PR will not alter performance

✅ 6 untouched benchmarks

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_{Comparing poljar/remove-encode-manual (562e055) with main (cfc4eee)}