Add dns.rdata.load_all_types().

[rthalley]

This PR provides dns.rdata.load_all_types(), which loads all rdata implementations instead of dynamically loading them on use. This is useful in some cases, as in issue [#1083] or if using chroot().

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 50.00000% with 5 lines in your changes missing coverage. Please review.

Project coverage is 93.91%. Comparing base (369a846) to head (788eddd).

Files	Patch %	Lines
dns/rdata.py	50.00%	5 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1095      +/-   ##
==========================================
- Coverage   93.92%   93.91%   -0.01%     
==========================================
  Files         143      143              
  Lines       13389    13396       +7     
  Branches     2607     2609       +2     
==========================================
+ Hits        12575    12581       +6     
- Misses        480      481       +1     
  Partials      334      334              

Flag	Coverage Δ
unittests	93.87% <50.00%> (-0.01%)	⬇️

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