Agents: add the memory feature to the library

[mrm1001]

Add a memory abstraction to keep track of messages when running a chat pipeline that interacts with a retriever.

User story

Users should be able to create a pipeline that can answer user queries by using previous queries, responses (raw messages), as well as documents from a document store.

It needs to support the following conversation patterns:

App has access to previous content to understand context.

User: what is the tallest building in the US?
App: The North Tower.
User: How tall is it?
App: It is 417m long.