Add an option to make useFieldArray's fields value reactive to form changes

[MWhite-22]

As the docs clearly state, no two useFieldArray for the same field can be instantiated, as each handles it's own snapshot of state internally.

However, when building large multi-page or multi-step forms, we like to lean in to hooks and context providers so we aren't passing a ton of props to deeply nested components. Being able to call a useFieldArray twice for the same field, so that different form pages can update the same field array, would be a nice quality of life improvement. 100% an "opt-in" feature, as I know it de-optimizes rerenders.

The way we handle it today is by creating our own react context and providing the results of any fieldArrays we want to use. It works, it's just a little boilerplate-y.

Example:

const FormFieldArrayCtx = React.createContext<CtxVal | null>(null);

export const FooFormProvider: React.FC<{
  children: React.ReactNode;
}> = ({ children }) => {
  const formMethods = useForm<FooForm>();

  const { control } = formMethods;
  const personsArray = useFieldArray({ control, name: 'people' });
  const transactionsArray = useFieldArray({ control, name: 'transactions' });

  const ctxVal = useMemo<CtxVal>(() => ({
    personsArray,
    transactionsArray,
  }), [personsArray, transactionsArray]);

  return (
    <FormProvider {...formMethods}>
      <FormFieldArrayCtx.Provider value={ctxVal}>
        {children}
      </FormFieldArrayCtx.Provider>
    </FormProvider>
  );
};

/**
 * Access any react-hook-form methods on the Foo Form
 */
export const useFooForm = () => {
  const methods = useFormContext<FooForm>();
  const fieldArrays = useContext(FormFieldArrayCtx);
  if (!fieldArrays) {
    throw new Error('useFooForm can only be used inside of a FooFormProvider');
  }
  return {
    ...methods,
    ...fieldArrays,
  };
};

Something like a reactive: true in the useFieldArray options would be a nice touch, and should be relatively simple to implement by having the internals of useFieldArray just subscribe/watch the values of the key it was provided. I took a look at source code and seems relatively simple, but I am interested in overall feedback/criticism of this approach.

Thanks!