[Data explorer] Add filtering on index pattern selector

[spapadop]

Is your feature request related to a problem? Please describe.

When having many index patterns in a cluster, the strategy of highlighting the typed text on the big list of index patterns is not ideal.
Previously, when typing something, the drop-down list would appear only index patterns that contain the typed text. Now it always show all of them, and just highlights the typed text.

Describe the solution you'd like

in this case, the typed text is at the very bottom of the list, so I need to scroll all the way down. Having a filtering mechanism instead of the highlighting one that exists now, it would show only that "monit_qa_alarm*" index pattern immediately, hiding all the rest irrelevant index patterns.

[wbeckler]

This looks like a Discover-related regression

[wbeckler]

This is a dupe of #5319

[wbeckler]

Please reopen if this doesn't seem correct.