Let the unspecified arguments of dict output of a callback function default to `dash.no_update` (or other user defined default)

If you use named arguments for the output a callback function, as in

@app.callback(
    output=dict(x=Output(...), y=Output(...)),
    inputs=dict(a=Input(...), b=Input(...)),
    state=dict(c=State(...))
)
def callback(b, c, a):
    return dict(x=a + b, y=b + c)

you have to specify all the arguments in the returning dict. But since named argument are useful especially when you have several output arguments, and if, like I experienced, you need to update only one or few arguments in the various return cases, it would be much more comfortable to return a dict specifying only those few arguments, instead of returning a long dict mostly made of dash.no_update.

I think the code would be shorter and clearer if the returning dict could default the unspecified named arguments to dash.no_update or some user defined value, like in defaultdict.

Start by tracing how dict outputs from callback functions are validated and normalized. Compare the existing dash.no_update behavior with omitted keys and the proposed user-defined default, then add focused tests for partial returns and default handling. Done means unspecified outputs consistently receive the selected default without changing explicitly returned values.