Add a `visible=true|false` flag to the HTTP API endpoints that return runners (`/user/actions/runners` and friends). Previously, all endpoints (except the one for admins) only returned the runners owned by the respective repository, user, or organization. The endpoint for admins returned all runners.
With this change, all endpoints only return the runners directly owned by the repository, user, organization, or instance by default (`visible=false`). With `visible=true`, the API returns the same runners as the UI. That means, for example, that `/repos/{owner}/{repo}/actions/runners?visible=true` returns all runners owned by the repository, its owner, and the instance.
Additionally, the behaviour of the endpoint for getting a single runner was altered. With this change, it permits accessing all _visible_ runners, thereby matching the UI. Previously, only runners directly owned by the repository, user, or organization could be obtained, whereas the admin could obtain all. Furthermore, existence probing is no longer possible.
## 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 for Go changes
(can be removed for JavaScript changes)
- 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 ran...
 - [x] `make pr-go` before pushing
### Tests for JavaScript changes
(can be removed for Go changes)
- 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.
- [x] I did not document these changes and I do not expect someone else to do it.
### Release notes
- [x] This change will be noticed by a Forgejo user or admin (feature, bug fix, performance, etc.). I suggest to include a release note for this change.
- [ ] This change is not visible to a Forgejo user or admin (refactor, dependency upgrade, etc.). I think there is no need to add a release note for this change.
*The decision if the pull request will be shown in the release notes is up to the mergers / release team.*
The content of the `release-notes/<pull request number>.md` file will serve as the basis for the release notes. If the file does not exist, the title of the pull request will be used instead.
Reviewed-on: forgejo/forgejo#11670
Reviewed-by: Mathieu Fenniak <mfenniak@noreply.codeberg.org>
Co-authored-by: Andreas Ahlenstorf <andreas@ahlenstorf.ch>
Co-committed-by: Andreas Ahlenstorf <andreas@ahlenstorf.ch>