[EuiSelectableTemplateSitewide] Interactive elements in popover footer are not keyboard accessible

[chandlerprall]

Once something is entered into the search field, we add a "View more results" button to the results popover footer. This isn't focusable via keyboard: arrow keys cycle only through the results and tab jumps to the search field's clear button.

Screen.Recording.2022-11-16.at.3.37.41.PM.mov

[daveyholler]

@1Copenut says this is a "sneaky large" since we'll have to consider the UX around what it means to "view results."

[daveyholler]

Related to: #6376

[cee-chen]

Tweaked the issue title to be a bit more generic and descriptive of the underlying issue. There are two potential approaches here: make the popover footer keyboard accessible through some kind of shortcut, or make it a UX advisory for consumers using this template not to put interactive elements in the footer (and fix our own docs to remove that behavior).