feat: add HTTP API endpoint for runner registration (#10677)

Add an HTTP API endpoint for runner registration. It enables managing the entire runner lifecycle using Forgejo's HTTP API. See https://code.forgejo.org/forgejo/forgejo-actions-feature-requests/issues/78 for background, design considerations, and usage.

Example usage:

```
$ curl -X POST -H "Content-Type: application/json" -H "Accept: application/json" -H "Authorization: token 3fc3ef39805b0f811a5d7789cb7b448348d6bfbb" --data '{"name":"api-runner","description":"Lorem ipsum"}' http://localhost:3000/api/v1/user/actions/runners
```
```json
{"id":30,"uuid":"a5e33697-9f58-437d-83c3-551b6c6a6334","token":"cac45fa6726fe4e28f42598773671af28a3be121"}
```

## 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.
- [x] 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/10677
Reviewed-by: Mathieu Fenniak <mfenniak@noreply.codeberg.org>
Co-authored-by: Andreas Ahlenstorf <andreas@ahlenstorf.ch>
Co-committed-by: Andreas Ahlenstorf <andreas@ahlenstorf.ch>

modules/structs/runner.go

@@ -0,0 +1,26 @@
+// Copyright 2025 The Forgejo Authors. All rights reserved.
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+package structs
+
+// RegisterRunnerOptions declares the accepted options for registering runners.
+// swagger:model
+type RegisterRunnerOptions struct {
+	// Name of the runner to register. The name of the runner does not have to be unique.
+	//
+	// required: true
+	Name string `json:"name" binding:"Required"`
+
+	// Description of the runner to register.
+	//
+	// required: false
+	Description string `json:"description"`
+}
+
+// RegisterRunnerResponse contains the details of the just registered runner.
+// swagger:model
+type RegisterRunnerResponse struct {
+	ID    int64  `json:"id" binding:"Required"`
+	UUID  string `json:"uuid" binding:"Required"`
+	Token string `json:"token" binding:"Required"`
+}