Add layout ref to access descriptor on OCI layout

[jonesbusy]

Description

Testing done

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

Attention: Patch coverage is 81.15942% with 13 lines in your changes missing coverage. Please review.

Project coverage is 84.73%. Comparing base (526739b) to head (fabca21).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/main/java/land/oras/OCILayout.java	71.73%	10 Missing and 3 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #202      +/-   ##
============================================
- Coverage     85.00%   84.73%   -0.28%     
- Complexity      374      395      +21     
============================================
  Files            31       33       +2     
  Lines          1274     1336      +62     
  Branches        141      145       +4     
============================================
+ Hits           1083     1132      +49     
- Misses          124      134      +10     
- Partials         67       70       +3     

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