Add EDE retrieval helper [#969].

[rthalley]

This adds a helper to extract EDE options from the EDNS0 options list. I think this is all we need to meet [#969], as resolver exceptions contain the message as well.

[bwelling]

This API looks reasonable, but I think it would be better if there were a more general version that returned all of the options of a specified type, and extended_errors was be a trivial wrapper around that. Right now, any code that wants to look at a single option type needs to iterate over all of the options.

[rthalley]

Good idea, and easily done.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (2ab3d16) 94.08% compared to head (52414ef) 94.08%.
Report is 1 commits behind head on main.

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Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1056   +/-   ##
=======================================
  Coverage   94.08%   94.08%           
=======================================
  Files         142      142           
  Lines       13141    13145    +4     
  Branches     2532     2533    +1     
=======================================
+ Hits        12364    12368    +4     
  Misses        463      463           
  Partials      314      314           

Flag	Coverage Δ
unittests	94.04% <100.00%> (+<0.01%)	⬆️

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