[EuiFilterGroup] Rebuild multi-select demo using EuiSelectable

[myasonik]

The existing EuiFilterGroup multi-select demo has a few a11y shortcomings and differs in implementation from EuiSelectable which has been crowned as the future implementation of dropdown-like things. To consolidate implementations of similar components, fix the a11y problems, and unify the UX, EuiFilterGroup should be rewritten using EuiSelectable.

On top of EuiSelectable, EuiFilterGroup's multi-select largely provides:

1. A custom button that open a popover (inside which EuiSelectable would/could live)

I think the EUI team prefers to keep the EuiComoboBox component and to wrap EuiSelectable within it but another implementation option is to add a "renderAs" prop (or something along those lines) to EuiSelectable to adjust styling.

[github-actions]

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