fix: update --add should add plain descriptor

[wangxiaoxuan273]

What this PR does / why we need it:

oras manifest index update --add now adds the original manifest descriptor, and which may include the annotations. The command should add the plain descriptor.

oras manifest index update --merge still adds the original manifest descriptors. The annotations may be meaningful under merge scenarios.

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 6 lines in your changes missing coverage. Please review.

Project coverage is 86.02%. Comparing base (304a89f) to head (955ade6).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
cmd/oras/root/manifest/index/create.go	60.00%	2 Missing and 2 partials ⚠️
cmd/oras/root/manifest/index/update.go	0.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1513   +/-   ##
=======================================
  Coverage   86.01%   86.02%           
=======================================
  Files         118      118           
  Lines        4225     4228    +3     
=======================================
+ Hits         3634     3637    +3     
+ Misses        353      352    -1     
- Partials      238      239    +1     

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[qweeah]

LGTM with minor suggestions

[qweeah]

The code coverage is hard to increase since #1509 is still WIP. After #1509 is done, we can add a unified add function for both index create and index update and test when config cannot be fetched. @wangxiaoxuan273