fuzz: add target for DecodeBase64

[inashivb]

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

Previous PR: https://github.com/OISF/suricata/pull/10955

Changes since v9:

• rebased

Note: Still haven't managed to run locally, making PR to avoid it going stale.

[codecov[bot]]

Codecov Report

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

Project coverage is 82.99%. Comparing base (c53e9ac) to head (8c5995c).

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #11002   +/-   ##
=======================================
  Coverage   82.99%   82.99%           
=======================================
  Files         917      917           
  Lines      248684   248684           
=======================================
+ Hits       206401   206404    +3     
+ Misses      42283    42280    -3     

Flag	Coverage Δ
fuzzcorpus	64.25% <ø> (+<0.01%)	:arrow_up:
suricata-verify	62.68% <ø> (+<0.01%)	:arrow_up:
unittests	62.32% <ø> (ø)

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[suricata-qa]

Information: QA ran without warnings.

Pipeline 20439