Support search_fields in DeepsetCloudDocumentStore

[tstadel]

Is your feature request related to a problem? Please describe.
For dense retrieval we can search multiple fields using EmbeddingRetriever.embed_meta_fields. For sparse this is possible via search_fields param that's offered by most document stores. DeepsetCloudDocumentStore is lacking of this option so far.

Describe the solution you'd like
Add search_fields param to DeepsetCloudDocumentStore

Describe alternatives you've considered
None.

Additional context
Add any other context or screenshots about the feature request here.