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>

services/actions/interface.go

@@ -31,6 +31,8 @@ type API interface {
 	ListRunners(*context.APIContext)
 	// GetRunner get a runner
 	GetRunner(*context.APIContext)
+	// RegisterRunner registers a new runner
+	RegisterRunner(*context.APIContext)
 	// DeleteRunner delete runner
 	DeleteRunner(*context.APIContext)
 }