Garbage collect OCI layout to cleanup non-referenced blobs

[jonesbusy]

Garbage collect OCI layout to cleanup non-referenced blobs

Fixes #263

Description

Testing done

mvn clean install

Submitter checklist

• I have read and understood the CONTRIBUTING guide
• I have run mvn license:update-file-header, mvn spotless:apply, pre-commit run -a, mvn clean install before opening the PR

[codecov]

Codecov Report

❌ Patch coverage is 86.95652% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.82%. Comparing base (398da85) to head (9766f11).

Files with missing lines	Patch %	Lines
src/main/java/land/oras/OCILayout.java	86.95%	3 Missing and 3 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #740      +/-   ##
============================================
+ Coverage     87.25%   87.82%   +0.56%     
- Complexity      985      994       +9     
============================================
  Files            45       45              
  Lines          3068     3113      +45     
  Branches        384      393       +9     
============================================
+ Hits           2677     2734      +57     
+ Misses          233      218      -15     
- Partials        158      161       +3     

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