feat: create index from existing manifests (index create)

[wangxiaoxuan273]

What this PR does / why we need it: oras manifest index create command

Current output:

~# oras manifest index create wxxdevreg.azurecr.io/test:tag0,tag1,tag2 amd64 armv7
Fetching  amd64
Fetched   amd64
Fetching  armv7
Fetched   armv7
Packed    5123c2f12c03  application/vnd.oci.image.index.v1+json
Pushed    [registry] wxxdevreg.azurecr.io/test:tag0
Tagged    tag2
Tagged    tag1
Digest: sha256:5123c2f12c03cf1af0fc7b27dde0e5c363fa61ecff7885a0fe6c09d1af4e039f

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

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?

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 88.33333% with 14 lines in your changes missing coverage. Please review.

Project coverage is 85.62%. Comparing base (3dc7eeb) to head (90f124c). Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
cmd/oras/root/manifest/index/create.go	84.94%	7 Missing and 7 partials :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1475 +/- ## ========================================== + Coverage 85.54% 85.62% +0.08% ========================================== Files 113 116 +3 Lines 3936 4056 +120 ========================================== + Hits 3367 3473 +106 - Misses 342 349 +7 - Partials 227 234 +7 ```

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