Add optional Milvus vector store backend

[zc277584121]

Summary

Adds an optional Milvus vector store backend for EvoAgentX RAG storage.

• Adds a LlamaIndex-compatible Milvus vector store backed by pymilvus.MilvusClient
• Keeps FAISS as the default vector store
• Adds vector_name="milvus" configuration fields for URI, token, database, collection, metric type, consistency level, and overwrite behavior
• Adds evoagentx[milvus] optional dependency with Milvus Lite support
• Documents Milvus configuration in the storage and RAG docs

Closes #266.

Validation

• ruff check evoagentx
• ruff check evoagentx/storages/vectore_stores tests/src/storages/test_milvus_vector_store.py
• pytest tests/src/storages/test_milvus_vector_store.py tests/src/storages/test_storagehandler.py -q
• Manual Milvus Lite + OpenAI text-embedding-3-small RAG smoke test
• Manual Zilliz Cloud direct vector store smoke test with a temporary collection

Notes

Milvus uses a local ./milvus.db URI by default for Milvus Lite. The same configuration can point to Milvus server or Zilliz Cloud by setting milvus_uri and milvus_token.

[zc277584121]

Hi @TedSIWEILIU, I opened this PR following the route discussed in #266. It keeps FAISS as the default, adds Milvus as an optional backend with lazy imports, and follows the existing VectorStoreBase interface. Would appreciate your review when you have time.