Add unique validator

[djhi]

Validating the uniqueness of a field is a common requirement so we'd like to provide the useUnique hook that returns a validator for this use case.

It will call the dataProvider.getList method with a filter to check whether a record exists with the current value of the input for the field matching the input source.

Basic usage (more options in the documentation):

const UserCreateForm = () => {
    const unique = useUnique();
    return (
        <SimpleForm>
            <TextInput source="username" validate={unique()} />
        </SimpleForm>
    );
};

[fzaninotto]

You should update the doc sidebar to add an entry for the new useUnique hook.