Fix BC break v4: useCreate and useUpdate can return a promise

[alanpoulain]

Related PR: #5778.

To be able to handle correctly submission validation errors, the returned functions from useCreate and useUpdate need to be able to return a promise.
Only then doing like it is described in the documentation would be possible:

react-admin/docs/CreateEdit.md

Lines 1524 to 1531 in efc8969

	try {
	await create('users', { data: values });
	} catch (error) {
	if (error.body.errors) {
	// The shape of the returned validation errors must match the shape of the form
	return error.body.errors;
	}
	}

[fzaninotto]

Thanks a lot!