feat: Workflow job based ephemeral runner scaling (#721)

This add support for two upcoming enhancements on the GitHub side of self-hosted runners, ephemeral runners, and `workflow_jow` events. You can't use these yet.

**These features are not yet generally available to all GitHub users**. Please take this pull request as a preparation to make it available to actions-runner-controller users as soon as possible after GitHub released the necessary features on their end.

**Ephemeral runners**:

The former, ephemeral runners, is basically the reliable alternative to `--once`, which we've been using when you enabled `ephemeral: true` (default in actions-runner-controller).

`--once` has been suffering from a race issue #466. `--ephemeral` fixes that.

To enable ephemeral runners with `actions/runner`, you give `--ephemeral` to `config.sh`. This updated version of `actions-runner-controller` does it for you, by using `--ephemeral` instead of `--once` when you set `RUNNER_FEATURE_FLAG_EPHEMERAL=true`.

Please read the section `Ephemeral Runners` in the updated version of our README for more information.

Note that ephemeral runners is not released on GitHub yet. And `RUNNER_FEATURE_FLAG_EPHEMERAL=true` won't work at all until the feature gets released on GitHub. Stay tuned for an announcement from GitHub!

**`workflow_job` events**:

`workflow_job` is the additional webhook event that corresponds to each GitHub Actions workflow job run. It provides `actions-runner-controller` a solid foundation to improve our webhook-based autoscale.

Formerly, we've been exploiting webhook events like `check_run` for autoscaling. However, as none of our supported events has included `labels`, you had to configure an HRA to only match relevant `check_run` events. It wasn't trivial.

In contrast, a `workflow_job` event payload contains `labels` of runners requested. `actions-runner-controller` is able to automatically decide which HRA to scale by filtering the corresponding RunnerDeployment by `labels` included in the webhook payload. So all you need to use webhook-based autoscale will be to enable `workflow_job` on GitHub and expose actions-runner-controller's webhook server to the internet.

Note that the current implementation of `workflow_job` support works in two ways, increment, and decrement. An increment happens when the webhook server receives` workflow_job` of `queued` status. A decrement happens when it receives `workflow_job` of `completed` status. The latter is used to make scaling-down faster so that you waste money less than before. You still don't suffer from flapping, as a scale-down is still subject to `scaleDownDelaySecondsAfterScaleOut `.

Please read the section `Example 3: Scale on each `workflow_job` event` in the updated version of our README for more information on its usage.

acceptance/testdata/repo.runnerset.yaml

@@ -18,7 +18,7 @@ spec:
   # From my limited testing, `ephemeral: true` is more reliable.
   # Seomtimes, updating already deployed runners from `ephemeral: false` to `ephemeral: true` seems to
   # result in queued jobs hanging forever.
-  ephemeral: false
+  ephemeral: ${TEST_EPHEMERAL}
 
   repository: ${TEST_REPO}
   #
@@ -52,5 +52,8 @@ spec:
       containers:
       - name: runner
         imagePullPolicy: IfNotPresent
+        env:
+        - name: RUNNER_FEATURE_FLAG_EPHEMERAL
+          value: "${RUNNER_FEATURE_FLAG_EPHEMERAL}"
       #- name: docker
       #  #image: mumoshu/actions-runner-dind:dev