[EuiSelectable] `aria-describedby` not set when used with `EuiFormRow`

**Describe the bug**

`EuiComboBox` [plays nicely with `EuiFormRow`](https://eui.elastic.co/docs/components/forms/selection/combo-box/#containers), which is useful and/or needed for accessibility reasons e.g.

```tsx

{comboBox}

```

That is not the case for `EuiSelectable`. `aria-describedby` is not being set by a wrapping `EuiFormRow` as consumers would expect.

**Impact and severity**

Severity is low, but fixing this should have high impact (as it would help with on-going accessibility efforts)

**Expected behavior**

That `aria-describedby` is properly added to the search input element when the `helpText` prop is set on a wrapping `EuiFormRow`.

**Related**

- https://github.com/elastic/kibana/issues/219379