[select] Make it possible to provide slotted list-box in HTML

[web-padawan]

Motivation

In V25, vaadin-select has slot="overlay" which should make it possible to pass list-box like this:

<vaadin-select value="foo">
  <vaadin-select-list-box slot="overlay">
    <vaadin-select-item value="foo">Foo</vaadin-select-item>
    <vaadin-select-item value="Bar">Bar</vaadin-select-item>
  </vaadin-select-list-box>
</vaadin-select>

Proposed solution

The logic needs to be modified so that instead of requestContentUpdate() we observe overlay slot content:

web-components/packages/select/src/vaadin-select-overlay-mixin.js

Lines 84 to 85 in 858c565

	const menuElement = this._getMenuElement();
	this.owner._assignMenuElement(menuElement);