use metadata filter for chats

[pmeier]

Feature description

See #422 for work that has to happen before this. See #256 for a discussion.

• SourceStorage.retrieve needs to take a MetadataFilter object as first input argument rather than a list of documents
• The builtin source storages need to be refactored in a way to only have a single index. The metadata of each document should be stored alongside the document in the DB. During retrieval, the metadata filter needs to be translated into the dialect of the DB
• The first input argument of Rag().chat() needs to be renamed to input and should allow a list of Documents as well as a MetadataFilter:
  • Passing a list of documents should retain the current behavior by storing them in the source storage and creating a metadata filter to select only these documents in the retrieval stage.
  • Passing a metadata filter should skip the storing and just set the prepared flag on the chat
    The cases of passing both types or none as explored in "managed" Ragna #256 (comment) will be saved for later. For now only a single input is fine
• For the REST API, instead of overloading one input parameter as done for the Python API, we should have two mutually exclusive documents and metadata_filter inputs.

Value and/or benefit

No response

Anything else?

No response