Add IsLoading to the field config

[MangelMaxime]

In a same way that a field can be disabled, I think adding IsLoading to the field config can make sense.

I am unsure if we should do it the field config level or if it should be at the Attributes level (per case by case).

I think slight in favour of doing it as the config level and if a type of field doesn't suppose loading then it can just ignore that property.

Doing it at the config level, imply that Fable.Form will have a Base.setLoading function in it like Base.disable.