Introduce 'defaultInputValue' input setting on dbjs model

[medikoo]

/cc @mtuchowski

We usually solve default via setting common value on prototype. Still such approach, makes property from beginning being filled with value, and that means we do not really require action from user (as value is already there)

There are cases where we want to provide this default value rather as hint, and still treat underlying value for property as null, so require from user a confirmation of an actual value that should be saved

We can solve it today by providing a value option directly to input configuration, but there's no option to provide such default directly via model