feat: update existing index (index update)

[wangxiaoxuan273]

What this PR does / why we need it:

oras manifest index update

Current status output:

root# oras manifest index update 
wxxdevreg.azurecr.io/test@sha256:2b8bab8c7d73d178fa66c391458e5a441e09ab47bb85c3b4936bd94ac07c9132 --add s390x --add sha256:89fa64341cccaf9b1ede4d06abf5217c785adb6cbb40827f7488db5d97f3d3c8 --remove sha256:a1f5af96aa0a2d2e77bb6f18174353d7fa96700a5fac76a9f460a44f6999330c --merge temp02 --tag temp03 --tag temp04
Fetching  sha256:2b8bab8c7d73d178fa66c391458e5a441e09ab47bb85c3b4936bd94ac07c9132
Fetched   sha256:2b8bab8c7d73d178fa66c391458e5a441e09ab47bb85c3b4936bd94ac07c9132
Fetching  s390x
Fetched   sha256:fd0201b8b0d7cdde2b9e8bcf5278b49668f1dd377a01ec1e2ef0e57382c25a0d s390x
Added     sha256:fd0201b8b0d7cdde2b9e8bcf5278b49668f1dd377a01ec1e2ef0e57382c25a0d s390x
Fetching  sha256:89fa64341cccaf9b1ede4d06abf5217c785adb6cbb40827f7488db5d97f3d3c8
Fetched   sha256:89fa64341cccaf9b1ede4d06abf5217c785adb6cbb40827f7488db5d97f3d3c8
Added     sha256:89fa64341cccaf9b1ede4d06abf5217c785adb6cbb40827f7488db5d97f3d3c8
Fetching  temp02
Fetched   sha256:a4c7b57cea489149bc30e7ecce464ea16ac44ea7dcf734ccfeadd4f6846b175c temp02
Merged    sha256:a4c7b57cea489149bc30e7ecce464ea16ac44ea7dcf734ccfeadd4f6846b175c temp02
Resolving sha256:a1f5af96aa0a2d2e77bb6f18174353d7fa96700a5fac76a9f460a44f6999330c
Resolved  sha256:a1f5af96aa0a2d2e77bb6f18174353d7fa96700a5fac76a9f460a44f6999330c
Removed   sha256:a1f5af96aa0a2d2e77bb6f18174353d7fa96700a5fac76a9f460a44f6999330c
Updated   sha256:79812a00bb50fc45e56e1744a5201a40364a417bfe444156f3209f3d451b7ecc
Pushed    [registry] wxxdevreg.azurecr.io/test@sha256:2b8bab8c7d73d178fa66c391458e5a441e09ab47bb85c3b4936bd94ac07c9132
Tagged temp04
Tagged temp03
Digest: sha256:79812a00bb50fc45e56e1744a5201a40364a417bfe444156f3209f3d451b7ecc

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged): Part of #1053

This PR builds on #1475

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?