Add a DavRequest extractor

[jwodder]

Closes #164.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 28 lines in your changes missing coverage. Please review.

Project coverage is 46.38%. Comparing base (0d53f0e) to head (ccb94af).
Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
src/dav/util.rs	0.00%	21 Missing ⚠️
src/dav/mod.rs	0.00%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #198      +/-   ##
==========================================
+ Coverage   46.33%   46.38%   +0.04%     
==========================================
  Files          29       29              
  Lines        3984     3939      -45     
==========================================
- Hits         1846     1827      -19     
+ Misses       2138     2112      -26     

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