Add NewCNAMESourceDescription helper method

[kevmo314]

Description

This specific source description is very common. This change introduces
a helper method to simplify the creation of CNAME source descriptions.

Reference issue

One part of #22

[codecov]

Codecov Report

Merging #111 (9495669) into master (a4508c4) will increase coverage by 0.17%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #111      +/-   ##
==========================================
+ Coverage   72.45%   72.62%   +0.17%     
==========================================
  Files          20       20              
  Lines        1285     1293       +8     
==========================================
+ Hits          931      939       +8     
  Misses        268      268              
  Partials       86       86              

Flag	Coverage Δ
go	72.62% <100.00%> (+0.17%)	⬆️
wasm	72.62% <100.00%> (+0.17%)	⬆️

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Impacted Files	Coverage Δ
source_description.go	78.44% <100.00%> (+1.59%)	⬆️

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