.NET: Add sample to show multiple AIContextProvider usage

[westey-m]

Motivation and Context

#2933

Description

• Adding a sample with an AggregateAIContextProvider that shows how to combine the AIContext from multiple providers together, and shows a couple of sample AIContextProviders at the same time.

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines
• All unit tests pass, and I have added new tests where possible
• Is this a breaking change? If yes, add "[BREAKING]" prefix to the title of the PR.

[Copilot]

Pull request overview

This PR adds a comprehensive sample demonstrating how to use multiple AIContextProvider instances with agents, addressing issue #2933. The sample shows how to aggregate multiple providers and demonstrates practical use cases including a todo list manager and calendar integration.

Changes:

• Added a new sample Agent_Step20_AdditionalAIContext that demonstrates AIContextProvider usage
• Implemented an AggregatingAIContextProvider to combine multiple context providers
• Updated the main README to reference the new sample

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 6 comments.

File	Description
dotnet/samples/GettingStarted/Agents/README.md	Added entry for the new Step 20 sample with description
dotnet/samples/GettingStarted/Agents/Agent_Step20_AdditionalAIContext/Program.cs	Main sample code implementing TodoListAIContextProvider, CalendarSearchAIContextProvider, and AggregatingAIContextProvider with demonstration usage
dotnet/samples/GettingStarted/Agents/Agent_Step20_AdditionalAIContext/Agent_Step20_AdditionalAIContext.csproj	Project file with necessary dependencies and configuration matching other samples
dotnet/agent-framework-dotnet.slnx	Added the new project to the solution file