dns: add dns.rcode keyword - v10

[jufajardini]

Feature #6621

Link to redmine ticket:
https://redmine.openinfosecfoundation.org/issues/6621

Previous PR: #10458

Describe changes from previous PR:

• rebase
• restructure commit message a bit (bring ticket number to last line, plus a couple of rewordings)
• restructure doc section a bit (remove double "the", use capital for Suricata, remove whitespace at end of lines, mention requests and responses on both example explanations...)

To use a pull request use a branch name like pr/N where N is the
pull request number.

SV_BRANCH=OISF/suricata-verify#1687

[codecov]

Codecov Report

Attention: Patch coverage is 92.92035% with 8 lines in your changes are missing coverage. Please review.

Project coverage is 82.69%. Comparing base (4afaadc) to head (f178db3).

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #10584      +/-   ##
==========================================
- Coverage   82.74%   82.69%   -0.06%     
==========================================
  Files         924      925       +1     
  Lines      247401   247514     +113     
==========================================
- Hits       204719   204675      -44     
- Misses      42682    42839     +157     

Flag	Coverage Δ
fuzzcorpus	64.02% <28.88%> (-0.09%)	⬇️
suricata-verify	61.89% <82.22%> (+0.01%)	⬆️
unittests	62.18% <71.68%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

[suricata-qa]

Information: QA ran without warnings.

Pipeline 18995

[inashivb]

LGTM 🚀

[victorjulien]

Merged in #10603, thanks!