Make it possible to show a loading state on inputs

[vsgoulart]

Is your feature request related to a problem? Please describe

In order to lazy load values for inputs we need to show a loading state on the inputs in case the request takes too long. For more context read #60

Describe the solution you'd like

The library could adopt the concept of a field state and make it possible for the user to change the state of each field:

form.setFieldState('fieldName', 'loading'); // states could be extended to other things like readOnly, etc

// or

form.setField('fieldName', { state: 'loading' });

Describe alternatives you've considered

We could also have a more generic approach like on Bootstrap input addons

const loadingSpinnerElement = createLoadingSpinner();
form.setFieldSuffixAddon('fieldName', loadingSpinnerElement);

// or

form.setField('fieldName', { suffixAddon: loadingSpinnerElement });