refactor!: modernize memory store and copy

[shizhMSFT]

What this PR does / why we need it

• This PR scopes to memory target and copy
• Modernize the code to utilize the latest feature of .NET 8.0 and C# 12 with nullable enabled
• Refactor the code structure to match the oras-go repository while maintaining the dotnet naming conventions
• Add ConfigureAwait(false) for library code
• Renamed MemoryTarget to MemoryStore as one of the content store.

Which issue(s) this PR resolves / fixes

Related PR: #87

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?

[codecov]

Codecov Report

Attention: 12 lines in your changes are missing coverage. Please review.

Comparison is base (6adeaaa) 72.24% compared to head (9cbbaca) 71.81%.

Files	Patch %	Lines
src/OrasProject.Oras/Content/MemoryStorage.cs	85.00%	2 Missing and 1 partial ⚠️
src/OrasProject.Oras/Content/MemoryStore.cs	82.35%	2 Missing and 1 partial ⚠️
src/OrasProject.Oras/Content/MemoryTagStore.cs	72.72%	2 Missing and 1 partial ⚠️
src/OrasProject.Oras/Extensions.cs	88.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #89      +/-   ##
==========================================
- Coverage   72.24%   71.81%   -0.44%     
==========================================
  Files          33       33              
  Lines        1027     1004      -23     
  Branches       91       88       -3     
==========================================
- Hits          742      721      -21     
  Misses        232      232              
+ Partials       53       51       -2     

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[Wwwsylvia]

LGTM

[Wwwsylvia]

Wait a moment. Something was not in sync... Let me review it again.

Reviewing it again.

[Wwwsylvia]

LGTM