.NET client for protocol

[brandonh-msft]

Addresses #62

This pull request introduces a .NET Client library, tests, and sample along with dependency update for those with known vulnerabilities.

Dependency Updates:

• Updated package references in samples/backend/csharp/ChatProtocolBackend.csproj to newer versions due to vulnerabilities in older ones.
• Added centralized package version management in sdk/dotnet/Directory.Packages.props.

Client Implementation:

• Implemented AiChatProtocolClient class in sdk/dotnet/Client/AIChatClient.cs to handle chat completions and file uploads.
• Created IAiChatProtocolClient interface in sdk/dotnet/Client/Interfaces/IAiChatProtocolClient.cs to define the contract for the chat client.
• Added AIChatClientOptions record in sdk/dotnet/Client/AIChatClientOptions.cs to configure the chat client.
• Client implementation directly references the models, protocol, etc. from the backend area of the project. Ideally, this would be refactored into a Protocol library, but I did not do that with this PR to avoid a large change.

Sample

• Added a sample Console App that shows how to configure & use AiChatProtocolClient object via Dependency Injection standards.

Test Coverage:

• Added unit tests for AiChatProtocolClient in sdk/dotnet/Tests/Client.Tests/AiChatProtocolClientTests.cs to verify functionality.
• Created test project configuration in sdk/dotnet/Tests/Client.Tests/Client.Tests.csproj and sdk/dotnet/Tests/Directory.Build.props. [1] [2]

Solution Configuration:

• Added solution file sdk/dotnet/dotnet.sln to include the client and test projects.