Fork support

[alpeshvas]

Summary
Forking a session support
A brief, one or two sentence explanation of the proposal.

We have been using at our Slack workspace and there is recurring usecase of branching off a new conversation from an original session from between.
Currently it jumbles up everything in one session when we want to drift to some other direction and maybe come back to from where we branched off.
Background & Motivation
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What are the problems that this RFC sets out to solve? Why now? Be brief!

It can be useful to illustrate your RFC as a user problem in this section.
(ex: "Users have reported that it is difficult to do X in Flue today.")

Goals
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Example
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