[EuiHeaderBreadcrumbs] Support secondary action for (last) breadcrumb

[cchaos]

In Kibana's background task initiative, the designs include adding an (icon) button to the end of the breadcrumbs list/last breadcrumb. Currently just placing an EuiButtonIcon in a breadcrumb causes some render issues.

For instance the text alignment is not quite right

And in the new Amsterdam theme, the styles are especially off.

Originally posted by @cchaos in elastic/kibana#83000 (comment)