Full-text MongoDBAtlas Retriever

[caiolang]

Hello! Thank you for your work on the Haystack integrations, they rock 🤘

Is your feature request related to a problem? Please describe.
For the moment there is only a MongoDBAtlasEmbeddingRetriever available, which blocks hybrid search on Haystack using MongoDB Atlas as a document store backend.

Describe the solution you'd like
Ideally, MongoDBAtlasDocumentStore class would be incremented with a _fulltext_retrieval() function (+ additional needed functions/properties) that would be called by a new MongoDBAtlasFullTextRetriever. This would return the results of a search with the standard MongoDB Atlas query API (using $search, as far as I understand).

Describe alternatives you've considered
Currently, the alternatives would be (1) to not use hybrid search at all, relying only on semantic search, or (2) hacking together an implementation of a full-text retriever by patching MongoDBAtlasDocumentStore and creating a MongoDBAtlasFullTextRetriever. None of these are ideal.

Additional context
N/A

Thanks for your attention reading this, and hope you consider it :)

[anakin87]

duplicate of #872