Allow updateInitialValue to take a model as optional parameter

[dc-p8]

Is your feature request related to a problem? Please describe.
I came across a case where I needed to update the initial value of my form but without updating the current model with the same instance.

Describe the solution you'd like
It would be great if updateInitialValue() could and optional model as parametter.
This should do the trick :

- options.updateInitialValue = () => (options._initialModel = clone(field.model));
+ options.updateInitialValue = (model = null) => (options._initialModel = clone(model ?? field.model));

Describe alternatives you've considered

(this.options as any)._initialModel = {
    ...this.model_initial,
    facturation: {
        ...this.model_initial.facturation,
        production_history: prod
    }
}
this.model = {
    ...this.model,
    facturation: {
        ...this.model.facturation,
        production_history: prod
    }
};

In this context, this.model_initial and this.model doesn't have the same value :

  this.store.pipe(select(/**/)).$.subscribe((v) => {
      const model_string = JSON.stringify(v); // avoid read-only store issues
      this.model = JSON.parse(model_string);
      this.model_initial = JSON.parse(model_string);
  });

[aitboudad]

PR welcomed!