[EuiBreadcrumbs] Add optional popover-style menu

[ryankeairns]

          @ryankeairns Quick reminder to move out the requested `EuiBreadcrumbs` behavior out of this issue and into a separate one - as of the next `EuiCollapsibleNavBeta` PR, my work on the collapsible nav itself will be complete and I'll be closing this issue.

Originally posted by @cee-chen in #6759 (comment)

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EuiBreadcrumbs Differences

Functional enhancements

• As part of our design explorations in the main navigation, we played with a few possibilities for where to place project links, cloud deployment links, and the space selector. We ultimately landed on the idea to prepend them as part of the breadcrumbs, as that fit very well hierarchically. Ideally, we'd like the ability to have these prepended breadcrumb items that will trigger a popover when clicked.