Allow broadcasting single value over props matched in ALL wildcard

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Is your feature request related to a problem? Please describe.
Sometimes I want to output the same value to every prop matched by the ALL wildcard (for example cleaning a form with Nones after submission). Right now I either have to hardcode the number of props, or use something like len(dash.ctx.outputs_list[0]) to obtain it, which instead hard-codes the order of the Output in the callback signature. Both of these approaches are prone to breakage when either the layout or callback signature changes. The second one also feels like an unnecessary boiler plate. More so, unnecessary data are transmitted.

Describe the solution you'd like
The output could be wrapped in something like a Broadcast wrapper, which would signal to dash that the outputted value should be broadcasted to every matched prop instead of being treated as an array with one value per prop.

Describe alternatives you've considered
New wildcard, perhaps called ALLBROADCAST, could be used for this. But this has the problem that it is not possible to chose whether to broadcast or not based on logic within the callback.

Additional context
None

No files, tests, or entry points are named in the issue. Start by tracing Dash's handling of callback outputs matched by the ALL wildcard, then define and test a callback-level way to broadcast one value without transmitting redundant data or depending on output order.