Add online terminal support for Kubernetes

What does this MR do?

Gives terminal access to kubernetes-deployed environments via the deployment service

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Part of idea to production

Screenshots (if relevant)

/root/reviewing/environments

/root/reviewing/environments/10013

/root/reviewing/enviroments/10013/terminal

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

• Closes #22864 (closed) #22958 (moved)
• Alternative to, and somewhat based on, !6770 (closed)
• Depends on gitlab-workhorse!83 (merged)