Create DocEmbedder class along with RAGRetriever

Currently, the `Document Embedder` comes separately and outside of Feast for RAG use cases. We are in the process of reviewing and merging the `RAGRetriever` PR https://github.com/feast-dev/feast/pull/5405 which currently does not explicitly add the Document Embedder as part of the RAGRetriever. Having this functionality within feast will help users using feast for RAG use case as a one stop solution.

As part of this implementation, we should:

Add the document embeddings capacity to feast using HuggingFace doc embedders OR any other embedders.
The RAGRetriever uses the DocEmbedder object for adding it to offline / online store.
The DocEmbedder provides the choice to choose HuggingFace doc embedders or any other.