Feature Request: Add Milvus Vector Store Integration for RAG

[zc277584121]

Description

Add Milvus as a vector database option for knowledge base RAG workflows, following the existing Spanner integration pattern.

Motivation

ADK currently supports Vertex AI RAG, LlamaIndex, and Spanner for vector-based retrieval. Milvus is a widely adopted open-source vector database purpose-built for AI applications. Adding Milvus support would give developers a lightweight, self-hosted (or cloud-managed via Zilliz) alternative for building RAG agents.

Proposed Solution

• MilvusVectorStore: Utility class for collection setup, data ingestion, and similarity search.
• MilvusToolset: BaseToolset implementation exposing a similarity_search tool to LLM agents.
• MilvusToolSettings / MilvusVectorStoreSettings: Pydantic configuration classes.
• Users provide their own embedding function (e.g., Google GenAI gemini-embedding-001).
• pymilvus>=2.5.0 as an optional dependency (google-adk[milvus]).

Related PR

#4417

[mertbozkir]

Nice PR dude! 👌

[secprog]

Nice work, It will fit better on https://github.com/google/adk-python-community @zc277584121