Add ARTIFACT_GROUP support for image_export.py

[rbdebeer-zz]

One line description of pull request

Adds support to export ARTIFACT_GROUP artifacts to image_export.py

Description:

Adds support to export ARTIFACT_GROUP artifacts to image_export.py. Please pay attention as I moved
the 'find_specs_per_source_type' from function local to class variable!

Related issue (if applicable): fixes #2015

Notes:

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Checklist:

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• No new new dependencies are required or l2tdevtools has been updated
• Reviewer assigned

[codecov]

Codecov Report

Merging #2166 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2166      +/-   ##
==========================================
+ Coverage   85.14%   85.15%   +0.01%     
==========================================
  Files         455      455              
  Lines       32037    32046       +9     
==========================================
+ Hits        27277    27290      +13     
+ Misses       4760     4756       -4

Impacted Files	Coverage Δ
plaso/engine/artifact_filters.py	91.76% <100%> (+3.6%)	⬆️
plaso/cli/image_export_tool.py	83.12% <0%> (+0.61%)	⬆️

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[joachimmetz]

Some minor nits, let's chat more about the need for a instance attribute and your thoughts for a cleaner approach.

[rbdebeer-zz]

PTAL

[joachimmetz]

LGTM, sry for the delay in review