Support for index subject

[jonesbusy]

Description

Support for index subject

Testing done

CI

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 95.65217% with 2 lines in your changes missing coverage. Please review.

Project coverage is 85.54%. Comparing base (5182d5e) to head (a96e627).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/main/java/land/oras/ManifestDescriptor.java	80.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #220      +/-   ##
============================================
+ Coverage     85.41%   85.54%   +0.12%     
- Complexity      453      458       +5     
============================================
  Files            33       33              
  Lines          1502     1515      +13     
  Branches        174      173       -1     
============================================
+ Hits           1283     1296      +13     
- Misses          144      145       +1     
+ Partials         75       74       -1     

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