ENHANCEMENT support for brettt89/silverstripe-garbage-collector

[muskie9]

• Suggest brettt89/silverstripe-garbage-collector package
• Adds GarbageCollectionTask to run the GarbageColelctorJob if available

resolves #129

[codecov]

Codecov Report

Merging #130 (09d948f) into master (ac5054f) will decrease coverage by 0.95%.
The diff coverage is 0.00%.

@@             Coverage Diff              @@
##             master     #130      +/-   ##
============================================
- Coverage     55.52%   54.57%   -0.96%     
- Complexity       86       88       +2     
============================================
  Files            14       15       +1     
  Lines           344      350       +6     
============================================
  Hits            191      191              
- Misses          153      159       +6     

Flag	Coverage Δ
unittests	54.57% <0.00%> (-0.96%)	⬇️

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Impacted Files	Coverage Δ
src/Task/GarbageCollectionTask.php	0.00% <0.00%> (ø)

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