add return_embedding parameter to OpenSearchEmbeddingRetriever

[nickprock]

Is your feature request related to a problem? Please describe.
In InMemoryEmbeddingRetriever there is the parameter return_embedding that returns also embedding in Document, with OpenSearchEmbeddingRetriever I must recalculates embeddings after the retrieve step.

[wochinge]

This would be actually helpful for us on deepset cloud side as well. No need to pull out the embeddings for us (basically just unneeded network traffic)

[nickprock]

Thanks @ArzelaAscoIi 🙏