Add support for DMARC 2.0 reports

[jgosmann]

These are only in draft state, but seemingly already sent in the wild. See #53.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.68%. Comparing base (90c3475) to head (8dce5f2).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #54      +/-   ##
==========================================
+ Coverage   96.28%   96.68%   +0.40%     
==========================================
  Files          30       33       +3     
  Lines        1828     1992     +164     
==========================================
+ Hits         1760     1926     +166     
+ Misses         68       66       -2     

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