feat: add AdditionalInterfaces and PreferredTransport props

[SergeyMenshykh]

This PR adds transport-related properties introduced in v0.2.4 of the A2A spec to the agent card.

Contributes to: #23

[gemini-code-assist]

Summary of Changes

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This pull request significantly updates the A2A .NET library by integrating new transport-related properties as defined in version 0.2.4 of the A2A specification. It introduces new data models to represent agent communication interfaces and transport protocols, enhancing the AgentCard's ability to describe an agent's connectivity. Additionally, it includes polyfills to ensure these new features are compatible with a wider range of .NET frameworks.

Highlights

• AgentCard Model Enhancement: I've updated the AgentCard.cs model to include two new properties: AdditionalInterfaces (a list of AgentInterface objects) and PreferredTransport (an AgentTransport object). These additions align the AgentCard with version 0.2.4 of the A2A specification, allowing agents to announce multiple supported transports and a preferred communication method.
• New AgentInterface Model: I've introduced a new AgentInterface.cs file, defining a class that encapsulates a target URL and its corresponding AgentTransport. This model is crucial for declaring specific endpoints and the transport protocols they support.
• New AgentTransport Value Type: I've added AgentTransport.cs, a new readonly struct that represents different transport protocols (e.g., JSONRPC). This struct provides value equality, ensures proper JSON serialization/deserialization, and includes a static JsonRpc property for convenience.
• C# Language Feature Polyfills: To ensure compatibility and enable the use of modern C# features like required members and init accessors across various .NET target frameworks, I've included polyfills for CompilerFeatureRequiredAttribute, IsExternalInit, and RequiredMemberAttribute under the Polyfills directory.
• Unit Tests for AgentTransport: I've added a new unit test file, AgentTransportTests.cs, to thoroughly validate the functionality of the AgentTransport struct, covering its constructor, equality comparisons (including case-insensitivity), ToString() method, and JSON serialization/deserialization.

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[gemini-code-assist]

Code Review

This PR introduces transport-related properties to the AgentCard, aligning with the A2A specification. Key feedback includes:

• Using a JsonConverter for AgentTransport to ensure correct JSON serialization.
• Adding URL validation to the Url property in AgentInterface.