Add a Developer Certificate of Origin

[bluesomewhere]

The Matrix.org Foundation is rolling out this DCO across all public repositories in its namespace. Of note, this DCO does not require the use of "real" or "legally identifiable" names.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.55%. Comparing base (9d90ff0) to head (69a7740).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #163      +/-   ##
==========================================
- Coverage   90.57%   90.55%   -0.02%     
==========================================
  Files          34       34              
  Lines        1909     1905       -4     
==========================================
- Hits         1729     1725       -4     
  Misses        180      180              

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[poljar]

I wonder if we should enforce this with a GitHub action https://github.com/apps/dco organization-wide.