[Feature][Integration][Java] add ElasticsearchVectorStore in Java

[twosom]

Linked issue: #321

Purpose of change

This PR implements the Java version of Vector Store functionality for Flink Agents, following the design proposal in #143. This implementation enables RAG (Retrieval-Augmented Generation) capabilities by providing vector-based context retrieval.

• Added Vector Store API with query support and document handling
• Added @VectorStore annotation for agent plan resource management
• Added context retrieval request and response events
• Implemented context retrieval action with vector store support
• Implemented Elasticsearch vector store integration
• Added ElasticsearchVectorStore RAG example with Ollama integration

Tests

API

Documentation

• [x] doc-needed
• [ ] doc-not-needed