feat: Implement Pack Manifests

[nhu1997]

What this PR does / why we need it

There are use cases where .net sdk is needed to support attaching annotations such as End of Life.

Which issue(s) this PR resolves / fixes

Pack Manifests · Issue #39 · oras-project/oras-dotnet (github.com)

Please check the following list

• Does the affected code have corresponding tests, e.g. unit test, E2E test?
• Does this change require a documentation update?
• Does this introduce breaking changes that would require an announcement or bumping the major version?
• Do all new files have an appropriate license header?

[sajayantony]

@nhu1997 could you fix the DCO errors?

[codecov]

Codecov Report

Attention: Patch coverage is 84.17722% with 25 lines in your changes missing coverage. Please review.

Project coverage is 81.03%. Comparing base (f33af92) to head (ca03e9e).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
....Oras/Exceptions/InvalidDateTimeFormatException.cs	0.00%	9 Missing ⚠️
...oject.Oras/Exceptions/InvalidMediaTypeException.cs	33.33%	6 Missing ⚠️
...ct.Oras/Exceptions/MissingArtifactTypeException.cs	33.33%	6 Missing ⚠️
src/OrasProject.Oras/Packer.cs	97.24%	2 Missing and 1 partial ⚠️
src/OrasProject.Oras/PackManifestOptions.cs	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #149      +/-   ##
==========================================
+ Coverage   80.48%   81.03%   +0.54%     
==========================================
  Files          30       35       +5     
  Lines         902     1060     +158     
  Branches      109      123      +14     
==========================================
+ Hits          726      859     +133     
- Misses        122      146      +24     
- Partials       54       55       +1     

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[shizhMSFT]

LGTM