Add support for form async validators + unique custom validator

[stissot]

Async Form Validators are useful to validate a form field value using a backend API.

This adds getFormAsyncValidators() similar to getFormValidators(), called by getFormConfig().

Is also adds the generic uniqueValidator() method returning an AsyncValidatorFn that can be used to check any model and field for existing values.

Usage in model (e.g Product.code):

    public getFormAsyncValidators(): FormAsyncValidators {
        return {
            code: [this.uniqueValidator('code')],
        };
    }

[sambaptista]

Validators are relative to forms, not to model or services and should be standalone. They should be in their specific class instead of NaturalAbstractModelService.

Can you refactor into an NaturalValidators.unique(args). The count function may be out of the class context but in the same file without export mention.

NaturalValidators.ts

// Not exported variable, not in class neither. It's like a private function for current file.
const count = (queryVariablesManager, name) => {
   ...
}

export class NaturalValidators {

    public static unique(fieldName: string, service: AbstractModelService<any, any, ...> , apollo: Apollo, name: string)   {
       ...
       count(...);
        ...
    }
}

Usage would be more standard

public getFormAsyncValidators(): FormAsyncValidators {
        return {
            code: [NaturalValidators.unique('code', this.userService, this.apollo, 'user')],
            numberField : [Validators.min(10)] // angular native validator usage for comparison
        };
}

[sambaptista]

I'm unsure about that "null". Don't change that for now, I'll test it by myself, and implement tests.

[sambaptista]

The rest seems ok, good job.

When the refactor is done, we can merge. We have to keep in mind that a debounce is still to do (in another commit/PR)

[sambaptista]

Shouldnt this be returned ?

[stissot]

Thanks for the feedback. I've relocated the validator in its own class NaturalValidators.unique(), but left the public count() method in NaturalAbstractModel, because it needs to access properties (name, apollo) from the actual model to generate the query, and it can also be used to count objects matching the QueryVariables for other purposes than the validator. Also added some tests.