WIP: Web terminal for CI builds

What does this MR do?

Enables web terminal for CI builds on Kubernetes. I wanted to create this WIP MR right away to help guide the feature discussion, taking implementation complexity into account.

What changes are missing in this MR?

• Delaying pod deletion for some time after the build completes:
  • Either for as long as the user is connected (upto a certain time limit) - if we want to provide web terminal for all running builds,
  • or, just for a fixed period - if we only enable it on explicit user request
    • in this case, we'll need a new column in the DB to store whether terminal access is enabled for a build
• Frontend changes with help from UX - for now I've just added a simple Terminal button in the right sidebar if the build is still running
• Refactoring of the controller code

I favour the first approach of enabling this feature for all running builds, unless it makes things more complex.

Why was this MR needed?

The most common use-case is to debug failed builds by interactively inspecting logs, partial build outputs, etc. It's something developers at my company frequently ask for.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

Closes #25990 (closed)

• Depends on gitlab-runner!841 (closed) and gitlab-workhorse!234 (merged)
• Old issue about adding web terminals for environments: https://gitlab.com/gitlab-org/gitlab-ce/issues/22864