Allow more resolver options

[jose-pr]

Some modifications to allow more resolver options, either by providing a resolver or multiple upstream servers.
Passed the tests, but maybe add more tests to check for multiple upstream resolvers or other desired configurations.
Added checks for modifying records by using a lock.
One main RecordsResolver to keep all the records logic in one place
Created a RoundRobinResolver to iterate through multiple resolvers until a answer is found
Allow listening in multiple ports

[jose-pr]

I did also take a look at more drastic changes, to fit some of my needs:
https://github.com/jose-pr/dnserver/tree/reorg
It mainly stays the same api but expands on configuration options and on better support for custom resolvers.

[codecov]

Codecov Report

Merging #19 (c973193) into main (a99dfe9) will decrease coverage by 6.48%.
The diff coverage is 84.42%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #19      +/-   ##
==========================================
- Coverage   97.54%   91.07%   -6.48%     
==========================================
  Files           4        5       +1     
  Lines         204      269      +65     
  Branches       37       62      +25     
==========================================
+ Hits          199      245      +46     
- Misses          3       12       +9     
- Partials        2       12      +10     

Files	Coverage Δ
dnserver/main.py	88.50% <84.42%> (-10.62%)	⬇️

... and 1 file with indirect coverage changes

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[jose-pr]

Was able to run the workflow on my fork and passing for this branch now