Add `tabindex` attribute support on Icon component to show BS4 popover on trigger `focus`

What does this MR do?

Backported changes from https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/6658.

Why was this MR needed?

This MR adds support for tabindex attribute on Icon Vue component to allow triggering BS4 Popovers via trigger type focus and dismiss on next click.

Does this MR meet the acceptance criteria?

• Tests added for this feature/bug
• Conforms to the code review guidelines
  • Has been reviewed by a Frontend maintainer

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ee/issues/6470