The existing EuiComboBox 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, EuiComboBox should be rewritten using EuiSelectable.

On top of EuiSelectable, EuiComboBox largely provides:

1. A clear button (which needs improved contrast from what's there today)
   Picture example

   (should be 4.5:1)
2. Buttons with the selected items inside the search field
3. A dropdown view of the results instead of inline
   Which currently has a minor a11y issue associated with it (Remove EuiComboBox down arrow button from tab order #5024)

   Popup indicator should be excluded from focus order (Under "Keyboard Interaction" for Combobox)

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.

This replaces Kibana issues #42988 and #39357

This issue fixes:

• [EuiCombobox][COGNITION]: Combo boxes should open on user interaction instead of focus event #5951
• [Accessibility] Regression in the Roles UI Combo Boxes kibana#27749
• Spaces ComboBox in Edit Role, screen reader has quirky behavior  kibana#136527
• [EuiCombobox] allow for multiline options of different height  #7712