Better support for Index artifact type

[jonesbusy]

Description

Better support for Index artifact type

Complete #590

Testing done

Rely on existing tests

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 94.44444% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 89.26%. Comparing base (9ddb42c) to head (ed731ca).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/main/java/land/oras/Index.java	94.44%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #594      +/-   ##
============================================
+ Coverage     88.28%   89.26%   +0.98%     
- Complexity      815      821       +6     
============================================
  Files            42       42              
  Lines          2415     2431      +16     
  Branches        297      299       +2     
============================================
+ Hits           2132     2170      +38     
+ Misses          172      151      -21     
+ Partials        111      110       -1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.