fuzz: add target for DecodeBase64

[inashivb]

Link to redmine ticket: https://redmine.openinfosecfoundation.org/issues/6050

Previous PR: https://github.com/OISF/suricata/pull/11002

Changes since v10:

• rebased

Note: Running 10+ mins on my system w/o any issues.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 83.65%. Comparing base (abb7424) to head (59bdfc0).

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #11032      +/-   ##
==========================================
+ Coverage   80.63%   83.65%   +3.02%     
==========================================
  Files         922      922              
  Lines      250137   250321     +184     
==========================================
+ Hits       201699   209411    +7712     
+ Misses      48438    40910    -7528     

Flag	Coverage Δ
fuzzcorpus	64.30% <ø> (+0.03%)	:arrow_up:
livemode	18.42% <ø> (-0.14%)	:arrow_down:
suricata-verify	62.75% <ø> (?)
unittests	62.26% <ø> (ø)

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

[suricata-qa]

Information: QA ran without warnings.

Pipeline 20537

[victorjulien]

@catenacyber I'll approve it if you think it's good.

[victorjulien]

Merged in #11072, thanks!