detect/ftp.command: Add sticky buffer

[jlucovsky]

Continuation of #12469

Issue: 7502

Add a sticky buffer for "ftp.command" for matching on FTP command names.

Link to ticket: https://redmine.openinfosecfoundation.org/issues/7502

Describe changes:

• Add a sticky buffer for ftp.command
• Briefly document ftp.command sticky buffer.

Updates

• Documentation: rule example is now syntactically valid.

Provide values to any of the below to override the defaults.

• To use an LibHTP, Suricata-Verify or Suricata-Update pull request,
  link to the pull request in the respective _BRANCH variable.
• Leave unused overrides blank or remove.

SV_REPO=
SV_BRANCH=OISF/suricata-verify#2245
SU_REPO=
SU_BRANCH=
LIBHTP_REPO=
LIBHTP_BRANCH=

[codecov]

Codecov Report

Attention: Patch coverage is 94.44444% with 2 lines in your changes missing coverage. Please review.

Project coverage is 80.39%. Comparing base (d63ad75) to head (6a943ae).
Report is 6 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #12478      +/-   ##
==========================================
- Coverage   80.52%   80.39%   -0.14%     
==========================================
  Files         923      924       +1     
  Lines      259176   259212      +36     
==========================================
- Hits       208708   208397     -311     
- Misses      50468    50815     +347     

Flag	Coverage Δ
fuzzcorpus	56.06% <50.00%> (-0.01%)	⬇️
livemode	19.39% <50.00%> (-0.03%)	⬇️
pcap	44.20% <50.00%> (+<0.01%)	⬆️
suricata-verify	63.08% <94.44%> (-0.25%)	⬇️
unittests	58.45% <50.00%> (+<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 24358

[victorjulien]

Merged in #12480, thanks!