docs: add an example of authentication with Azure Container Registry

[wangxiaoxuan273]

What this PR does / why we need it

This PR adds an example of using oras-dotnet to authenticate with Azure Container Registry.

Which issue(s) this PR resolves / fixes

Resolves / Fixes #267

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

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.76%. Comparing base (84309e7) to head (7ae9386).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #321   +/-   ##
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  Coverage   91.76%   91.76%           
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  Files          64       64           
  Lines        2755     2755           
  Branches      364      364           
=======================================
  Hits         2528     2528           
  Misses        138      138           
  Partials       89       89           

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[Copilot]

Pull request overview

This PR adds documentation and example code demonstrating how to authenticate with Azure Container Registry (ACR) using Azure Active Directory (AAD) credentials in the ORAS .NET library.

Changes:

• Added AzureCredentialProvider class implementing ICredentialProvider for ACR authentication via AAD token exchange
• Added AzureContainerRegistryAuthentication example demonstrating ACR authentication and cross-registry copy operations
• Added Azure SDK dependencies (Azure.Containers.ContainerRegistry 1.3.0, Azure.Identity 1.13.1)
• Updated API documentation index to include the new ACR authentication example

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 12 comments.

Show a summary per file

File	Description
tests/OrasProject.Oras.Tests/examples/AzureCredentialProvider.cs	Implements ICredentialProvider for ACR authentication using Azure DefaultAzureCredential and AAD token exchange
tests/OrasProject.Oras.Tests/examples/AzureContainerRegistryAuthentication.cs	Demonstrates usage of AzureCredentialProvider for cross-registry artifact copy operations
tests/OrasProject.Oras.Tests/OrasProject.Oras.Tests.csproj	Adds Azure SDK package references for ACR and Azure Identity support
docs/api/index.md	Adds link to the new ACR authentication documentation
docs/api/authenticate_with_ACR.md	Documentation page that includes the ACR authentication examples

[Copilot]

Pull request overview

Copilot reviewed 5 out of 5 changed files in this pull request and generated no new comments.

[Copilot]

Pull request overview

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

[Copilot]

Pull request overview

Copilot reviewed 6 out of 7 changed files in this pull request and generated 1 comment.

[Copilot]

Pull request overview

Copilot reviewed 6 out of 8 changed files in this pull request and generated 2 comments.

[Copilot]

Pull request overview

Copilot reviewed 6 out of 6 changed files in this pull request and generated 3 comments.

[Copilot]

Pull request overview

Copilot reviewed 6 out of 6 changed files in this pull request and generated 3 comments.