feat: Add ChatMessage store support (including reader+writer)

[vblagoje]

Why:

Enhances Haystack chat use cases by adding an in-memory store for chat messages along with components for message retrieval and writing.

What:

• Added InMemoryChatMessageStore to store chat messages in memory.
• Introduced ChatMessageStore protocol to define a standard interface for chat message storage.
• Created ChatMessageRetriever and ChatMessageWriter components for fetching and storing chat messages, respectively.
• Extended test coverage with unit tests for the new chat message store, retriever, and writer components.

How can it be used:

The following examples demonstrate how to use the newly added components:

• Retrieving Chat Messages:

  from haystack_experimental.components.retrievers import ChatMessageRetriever
  from haystack_experimental.chat_message_stores.in_memory import InMemoryChatMessageStore
  
  store = InMemoryChatMessageStore()
  
  retriever = ChatMessageRetriever(message_store=store)
  retrieved_messages = retriever.run()["messages"]

• Writing Chat Messages:

  from haystack_experimental.components.writers import ChatMessageWriter
  from haystack_experimental.chat_message_stores.in_memory import InMemoryChatMessageStore
  
  store = InMemoryChatMessageStore()
  
  writer = ChatMessageWriter(message_store=store)
  new_messages = [ChatMessage.from_user("I have a question about Python.")]
  writer.run(messages=new_messages)

• Colab notebook (pipeline usage):

  • Here is the notebook demoing these classes in a concrete chat+RAG use case

How did you test it:

• Implemented unit tests for InMemoryChatMessageStore to ensure proper initialization, message storage behaviors, and serialization/deserialization functionality.
• Added unit tests for ChatMessageRetriever and ChatMessageWriter to validate interaction with the message store, including message retrieval and writing capabilities.
• Validated all tests to confirm they pass and accurately reflect component behaviors.

Notes for the reviewer:

• The ChatMessageRetriever of this initial version does not have filters and top_k run method parameters intentionally, as they will be added in subsequent PR.
• Please review the implementation of the ChatMessageStore protocol hierarchy to ensure it aligns well with our design principles

[coveralls]

Pull Request Test Coverage Report for Build 10147173400

Details

• 221 of 236 (93.64%) changed or added relevant lines in 12 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.3%) to 91.132%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
haystack_experimental/chat_message_stores/in_memory/chat_message_store.py	23	24	95.83%
haystack_experimental/components/retrievers/chat_message_retriever.py	30	34	88.24%
haystack_experimental/components/writers/chat_message_writer.py	31	35	88.57%
haystack_experimental/chat_message_stores/types/protocol.py	12	18	66.67%

Totals
Change from base Build 10145832341:	0.3%
Covered Lines:	1973
Relevant Lines:	2165

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💛 - Coveralls

[julian-risch]

@vblagoje Could you please pick a different reviewer for this as I was involved in writing this?
I would suggest to separate the store, retriever and writer into separate PRs. Makes it easier to review. ~700 lines are a lot.

[vblagoje]

Ok deal @julian-risch