Security and SecuritySchemes sample

Because when implementing and deploying the application with the A2A protocol, we normally need authentication/authorization.

I go to https://github.com/a2aproject/a2a-dotnet/blob/main/src/A2A/Models/AgentCard.cs#L88, https://github.com/a2aproject/a2a-dotnet/blob/main/src/A2A/Models/AgentCard.cs#L94, and the unit test at https://github.com/a2aproject/a2a-dotnet/blob/main/tests/A2A.UnitTests/Models/AgentCardTests.cs#L61 doesn't reflect how we can use these attributes in applications using A2A.

That would be great if we could have an example or guidance on how to do authentication/authorisation with the A2A protocol.