.NET: Add sample to show how to do RAG using Foundry's built-in service (#2324)

* Add sample to show how to do RAG using Foundry's built-in service

* Update README.me

* Update dotnet/samples/GettingStarted/AgentWithRAG/AgentWithRAG_Step04_FoundryServiceRAG/Program.cs

Co-authored-by: Roger Barreto <19890735+rogerbarreto@users.noreply.github.com>

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Co-authored-by: Roger Barreto <19890735+rogerbarreto@users.noreply.github.com>

dotnet/samples/GettingStarted/AgentWithRAG/README.md

@@ -7,3 +7,4 @@ These samples show how to create an agent with the Agent Framework that uses Ret
 |[Basic Text RAG](./AgentWithRAG_Step01_BasicTextRAG/)|This sample demonstrates how to create and run a basic agent with simple text Retrieval Augmented Generation (RAG).|
 |[RAG with Vector Store and custom schema](./AgentWithRAG_Step02_CustomVectorStoreRAG/)|This sample demonstrates how to create and run an agent that uses Retrieval Augmented Generation (RAG) with a vector store. It also uses a custom schema for the documents stored in the vector store.|
 |[RAG with custom RAG data source](./AgentWithRAG_Step03_CustomRAGDataSource/)|This sample demonstrates how to create and run an agent that uses Retrieval Augmented Generation (RAG) with a custom RAG data source.|
+|[RAG with Foundry VectorStore service](./AgentWithRAG_Step04_FoundryServiceRAG/)|This sample demonstrates how to create and run an agent that uses Retrieval Augmented Generation (RAG) with the Foundry VectorStore service.|