Allow a hook for `custom_data` into the `callback_context`

[BSd3v]

Would like for the ability to set custom_data inside of callback_context. This would give the flexibility to establish a variable that can be accessed by any function done within the context of a callback. Including background callbacks to grant access to things like user settings, etc.

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@gvwilson would be good to update this part in the docs too:
https://dash.plotly.com/advanced-callbacks#properties-for-callback_context

ie
ec3a35f

[BSd3v]

merged in #3175