feat: implement ephemeral runners (#9962)

As described in [this comment](https://gitea.com/gitea/act_runner/issues/19#issuecomment-739221) one-job runners are not secure when running in host mode. We implemented a routine preventing runner tokens from receiving a second job in order to render a potentially compromised token useless. Also we implemented a routine that removes finished runners as soon as possible.

Big thanks to [ChristopherHX](https://github.com/ChristopherHX) who did all the work for gitea!

Rel: #9407

## Checklist

The [contributor guide](https://forgejo.org/docs/next/contributor/) contains information that will be helpful to first time contributors. There also are a few [conditions for merging Pull Requests in Forgejo repositories](https://codeberg.org/forgejo/governance/src/branch/main/PullRequestsAgreement.md). You are also welcome to join the [Forgejo development chatroom](https://matrix.to/#/#forgejo-development:matrix.org).

### Tests

- I added test coverage for Go changes...
  - [ ] in their respective `*_test.go` for unit tests.
  - [x] in the `tests/integration` directory if it involves interactions with a live Forgejo server.
- I added test coverage for JavaScript changes...
  - [ ] in `web_src/js/*.test.js` if it can be unit tested.
  - [ ] in `tests/e2e/*.test.e2e.js` if it requires interactions with a live Forgejo server (see also the [developer guide for JavaScript testing](https://codeberg.org/forgejo/forgejo/src/branch/forgejo/tests/e2e/README.md#end-to-end-tests)).

### Documentation

- [ ] I created a pull request [to the documentation](https://codeberg.org/forgejo/docs) to explain to Forgejo users how to use this change.
- [ ] I did not document these changes and I do not expect someone else to do it.

### Release notes

- [ ] I do not want this change to show in the release notes.
- [ ] I want the title to show in the release notes with a link to this pull request.
- [ ] I want the content of the `release-notes/<pull request number>.md` to be be used for the release notes instead of the title.

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/9962
Reviewed-by: Andreas Ahlenstorf <aahlenst@noreply.codeberg.org>
Reviewed-by: Mathieu Fenniak <mfenniak@noreply.codeberg.org>
Co-authored-by: Manuel Ganter <manuel.ganter@think-ahead.tech>
Co-committed-by: Manuel Ganter <manuel.ganter@think-ahead.tech>

tests/integration/fixtures/TestEphemeralRunner/action_run.yml

@@ -0,0 +1,61 @@
+-
+  id: 10895
+  title: "running workflow_dispatch run"
+  repo_id: 64
+  owner_id: 3
+  workflow_id: "running.yaml"
+  index: 3
+  trigger_user_id: 2
+  ref: "refs/heads/main"
+  commit_sha: "c2d72f548424103f01ee1dc02889c1e2bff816b0"
+  event: "workflow_dispatch"
+  trigger_event: "workflow_dispatch"
+  is_fork_pull_request: 0
+  status: 6 # running
+  started: 1683636528
+  created: 1683636108
+  updated: 1683636626
+  need_approval: 0
+  approved_by: 0
+  event_payload: '{}'
+
+-
+  id: 10896
+  title: "running workflow_dispatch run"
+  repo_id: 64
+  owner_id: 3
+  workflow_id: "running.yaml"
+  index: 4
+  trigger_user_id: 2
+  ref: "refs/heads/main"
+  commit_sha: "c2d72f548424103f01ee1dc02889c1e2bff816b0"
+  event: "workflow_dispatch"
+  trigger_event: "workflow_dispatch"
+  is_fork_pull_request: 0
+  status: 6 # running
+  started: 1683636528
+  created: 1683636108
+  updated: 1683636626
+  need_approval: 0
+  approved_by: 0
+  event_payload: '{}'
+
+-
+  id: 10897
+  title: "waiting workflow for user runner scope test"
+  repo_id: 64
+  owner_id: 10
+  workflow_id: "waiting.yaml"
+  index: 5
+  trigger_user_id: 10
+  ref: "refs/heads/main"
+  commit_sha: "c2d72f548424103f01ee1dc02889c1e2bff816b0"
+  event: "push"
+  is_fork_pull_request: 0
+  status: 5 # waiting
+  started: 0
+  created: 1683636108
+  updated: 1683636108
+  need_approval: 0
+  approved_by: 0
+  event_payload: '{"head_commit":{"id":"c2d72f548424103f01ee1dc02889c1e2bff816b0"}}'