Add flag to disable scaling scores to probabilities

[tstadel]

Proposed changes:

• add scale_score flag to retrievers
• propagate scale_score down to document stores' query_by_embedding
• consolidate usage of scaling function throughout document stores
• adjust docstrings of Document's score reflecting the possibility of non-unit-interval values

Usage:

• as plain retriever:

EmbeddingRetriever(
        document_store=document_store_with_docs, embedding_model="sentence-transformers/paraphrase-MiniLM-L3-v2", scale_score=False
    )

• via pipeline params:

pipeline.run(query="who am I?", params={"scale_score": False})

Status (please check what you already did):

• First draft (up for discussions & feedback)
• Final code
• Added tests
• Updated documentation

[julian-risch]

Let's check milvus support of cosine similarity and the changes in haystack/document_stores/milvus2.py with @MichelBartels before merging. All other changes look good to me. 👍