Implemented appending arbitrary messages

[eavanvalkenburg]

Implemented appending arbitrary messages to the base chat message history, the in-memory and cosmos ones.

As discussed this is the alternative way instead of #4480, with a add_message method added that takes a BaseMessage as input, so that the user can control what is in the base message like kwargs.

Fixes # (issue)

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@hwchase17

[eyurtsev]

@hwchase17 -- Proposal seems to add an abstract method to add a custom message which seems reasonable to me -- it looks like a more appropriate method for the base schema than add_ai_message or add_human_message. Waiting for your :eye:

[eavanvalkenburg]

Thanks @hwchase17

[eavanvalkenburg]

@amitmukh this has been merged and release and will allow you to override the regular append_message behaviour