Add InMemoryChatMessageStore in haystack-experimental

[julian-risch]

Add an InMemoryChatMessageStore similar to InMemoryDocumentStore that can work with ChatMessageRetriever and ChatMessageWriter.

[julian-risch]

Here is a branch with InMemoryChatMessageStore: https://colab.research.google.com/drive/1eDGOR241q9Ek35BvETp1ihiA5yJH9dkK?usp=sharing
I merged the ChatMessageWriter and ChatMessageRetriever and here is an example notebook using the three: deepset-ai/haystack-experimental@main...chat_message_store

Should be separated in case we open PRs. Right now the purpose of the branches is to facilitate a discussion about alternative implementations of memory

Cc @vblagoje

[vblagoje]

Should we create RAG+chat to accent the difference between the old and the new approach? I can do that part...

[julian-risch]

closed by deepset-ai/haystack-experimental#49