Possibility to forward a change to a subset of clients (conditional broadcast)

[FlorianJacta]

Description
Being able to broadcast variables not to all the clients but only a subset. A user variable or filter function defines this subset.

Acceptance Criteria

• Ensure new code is unit tested, and check code coverage is at least 90%
• Propagate any change on the demos and run all of them to ensure there is no breaking change
• Ensure any change is well documented

[FlorianJacta]

I think we have internally a _broadcast function that broadcasts to a list of clients. Is it true?

[github-actions]

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[FabienLelaquais]

A great use case is a Chat application: not all clients should receive a message sent to a subset of connected users.