Add keep_alive_slots component

• Requirements (issue)
• Tasks (epic)
• Full Context Spike MR (permanent draft)
• All MR parts ...
  • Add keep_alive_slots component
    • Frontend for top nav menu redesign (depends on parent MR)

What does this MR do?

This MR introduces a general component that is used by the upcoming Top Nav redesign. You can see how it's used in this commit:

https://gitlab.com/gitlab-org/gitlab/-/blob/523620a571069a03265273d8423bf09f0790f057/app/assets/javascripts/nav/components/top_nav_dropdown_menu.vue#L120

Does this MR meet the acceptance criteria?

Conformity

• [ ] I have included a changelog entry, or it's not needed. (Does this MR need a changelog?) Not needed because no user-facing change.
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.