refactor: Move whois to client pkg, impl caching and update DNS used for tests

[TwiN]

Summary

Checklist

• Tested and/or added tests to validate that the changes work as intended, if applicable.
• Added the documentation in README.md, if applicable.

[codecov-commenter]

Codecov Report

Base: 82.02% // Head: 81.88% // Decreases project coverage by -0.13% ⚠️

Coverage data is based on head (8e89851) compared to base (861c443).
Patch coverage: 64.51% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #366      +/-   ##
==========================================
- Coverage   82.02%   81.88%   -0.14%     
==========================================
  Files          54       54              
  Lines        4028     4047      +19     
==========================================
+ Hits         3304     3314      +10     
- Misses        569      576       +7     
- Partials      155      157       +2     

Impacted Files	Coverage Δ
core/endpoint.go	74.57% <33.33%> (-1.36%)	⬇️
client/client.go	58.26% <70.83%> (+2.92%)	⬆️
config/config.go	73.82% <100.00%> (ø)

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