[FEATURE] Add name to dev.langchain4j.rag.content.retriever.EmbeddingStoreContentRetriever to improve logs

[alwa]

Is your feature request related to a problem? Please describe.
Log messages today are not very nice when using multiple retrievers (to see which one is used):

2024-04-18T19:19:18.931+02:00 DEBUG 62320 --- [pool-2-thread-1] d.l.rag.DefaultRetrievalAugmentor : Routing query '' to the following retriever: dev.langchain4j.rag.content.retriever.EmbeddingStoreContentRetriever@965f916

Describe the solution you'd like

1. Add name property to EmbeddingStoreContentRetriever
2. Implement EmbeddingStoreContentRetriever.toString() and use the name property in that implementation