[EuiSelectable] Support `isInvalid` prop

**Problem Statement**

It's not possible to correctly add `aria-invalid` to the elements of `EuiSelectable`, the search and the list. For the search field it's possible to use the `searchProps` object to pass it down; for the list is not possible to add it to the `

` elemenet.

This is necessary to properly support announcing errors with screen readers.

Will also help resolve https://github.com/elastic/kibana/issues/219379

**Proposed Solution**

Add a top-level `isInvalid` prop to `EuiSelectable` that will add the `aria-invalid` attribute to the corresponding elements.

**Use Case**

Announcing errors with screen readers correctly, without workarounds (e.g. https://github.com/elastic/kibana/issues/219379)

**Value / Impact**

It's needed for accessibility.

**Do alternatives or workarounds exist?**

Yes (see https://github.com/elastic/kibana/issues/219379), but they're cumbersome, and the fix is a very small effort.

**Additional context (Optional)**

It was requested in Slack.