Add VertexAI RAG retrieval tool implementation

[nktks]

• This PR adds Vertex AI RAG Engine tool https://google.github.io/adk-docs/tools/built-in-tools/#vertex-ai-rag-engine.
• This is based on adk-python implementation
  • However I omitted the implementation of functional tool implementation with google.cloud.aiplatform.v1.VertexRagService.RetrieveContexts call for Gemini 1.x support, because Gemini 1.x is already retired before adk-go had released. ref

[gemini-code-assist]

Summary of Changes

Hello @nktks, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request integrates the Vertex AI RAG (Retrieval-Augmented Generation) Engine tool into the adk-go framework, enabling agents to perform Retrieval-Augmented Generation by querying specified RAG corpora. The changes introduce a new VertexAIRAG tool that configures LLM requests to utilize Vertex AI's built-in RAG capabilities, specifically for Gemini models. An accompanying example demonstrates its usage, and dedicated tests ensure its proper functioning.

Highlights

• Vertex AI RAG Tool Integration: Introduces the VertexAIRAG tool for Retrieval-Augmented Generation using Vertex AI, enabling agents to leverage external knowledge bases.
• Built-in RAG Support: The implementation specifically utilizes the built-in Vertex AI RAG tool for Gemini models, intentionally omitting older Gemini 1.x functional tool support due to its retirement.
• New Example for Usage: Adds a new example file, examples/tools/retrievaltool/main.go, which demonstrates how to configure and use the VertexAIRAG tool within an llmagent.
• Core Tool Implementation: Defines the VertexAIRAG struct and its ProcessRequest method in tool/retrievaltool/vertex_ai_rag.go, responsible for injecting the RAG configuration into LLM requests.
• Comprehensive Testing: Includes tool/retrievaltool/vertex_ai_rag_test.go to validate the correct integration and functionality of the VertexAIRAG tool, ensuring it properly modifies LLM requests.

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[gemini-code-assist]

Code Review

This pull request introduces a new retrieval tool for Vertex AI RAG, along with an example and tests. The core implementation is clean and follows the existing tool pattern. My review focuses on improving the reusability of the example code and enhancing the thoroughness of the tests. I've suggested making configuration parameters in the example readable from environment variables and expanding test coverage to include all fields of the RAG configuration. I also found and suggested a fix for a potential panic in the new test case.