feat: oras manifest index: artifactType for index and child manifest descriptors

[apparentlymart]

What this PR does / why we need it:

This implements the new behavior proposed in https://github.com/oras-project/oras/pull/1696.

I am contributing this change on behalf of the OpenTofu project.

Which issue(s) this PR fixes:

Fixes #1670

Please check the following list:

• [x] Does the affected code have corresponding tests, e.g. unit test, E2E test?
• [x] Does this change require a documentation update?
• [ ] Does this introduce breaking changes that would require an announcement or bumping the major version?
• [x] Do all new files have an appropriate license header? (no new files)

[apparentlymart]

(I'm going to leave this as a draft for now because https://github.com/oras-project/oras/pull/1696 is not finalized yet.)

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 85.24%. Comparing base (b28a30b) to head (c2676f5). Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1700      +/-   ##
==========================================
+ Coverage   85.20%   85.24%   +0.03%     
==========================================
  Files         129      129              
  Lines        5780     5815      +35     
==========================================
+ Hits         4925     4957      +32     
- Misses        609      611       +2     
- Partials      246      247       +1     

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

:rocket: New features to boost your workflow:

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