[Feature][integration][Java] Java Implementation of Vector Store with Elasticsearch Support

[twosom]

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Description

Following the design proposal in #143, we need to implement Java version of Vector Store functionality to enable RAG (Retrieval-Augmented Generation) capabilities in Flink Agents. This issue focuses on implementing the core Vector Store infrastructure and Elasticsearch-based vector store as the first implementation.

Are you willing to submit a PR?

• I'm willing to submit a PR!

[xintongsong]

Created #385 for the documentation. Will keep the issue open until documentation is completed.