Add health check endpoint (#18465)

* chore: add health check endpoint

docs: update document about health check

fix: fix up Sqlite3 ping. current ping will success even if the db file is missing

fix: do not expose privacy information in output field

* refactor: remove HealthChecker struct

* Added `/api/healthz` to install routes.

This was needed for using /api/healthz endpoint in Docker healthchecks,
otherwise, Docker would never become healthy if using healthz endpoint
and users would not be able to complete the installation of Gitea.

* Update modules/cache/cache.go

* fine tune

* Remove unnecessary test code. Now there are 2 routes for installation (and maybe more in future)

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: 6543 <6543@obermui.de>
Co-authored-by: Marcos de Oliveira <marcossantos@furb.br>

docs/content/doc/installation/on-kubernetes.en-us.md

@@ -25,3 +25,47 @@ helm install gitea gitea-charts/gitea
 ```
 
 If you would like to customize your install, which includes kubernetes ingress, please refer to the complete [Gitea helm chart configuration details](https://gitea.com/gitea/helm-chart/)
+
+## Health check endpoint
+
+Gitea comes with a health check endpoint `/api/healthz`, you can configure it in kubernetes like this:
+
+```yaml
+  livenessProbe:
+    httpGet:
+      path: /api/healthz
+      port: http
+    initialDelaySeconds: 200
+    timeoutSeconds: 5
+    periodSeconds: 10
+    successThreshold: 1
+    failureThreshold: 10
+```
+
+a successful health check response will respond with http code `200`, here's example:
+
+```
+HTTP/1.1 200 OK
+
+
+{
+  "status": "pass",
+  "description": "Gitea: Git with a cup of tea",
+  "checks": {
+    "cache:ping": [
+      {
+        "status": "pass",
+        "time": "2022-02-19T09:16:08Z"
+      }
+    ],
+    "database:ping": [
+      {
+        "status": "pass",
+        "time": "2022-02-19T09:16:08Z"
+      }
+    ]
+  }
+}
+```
+
+for more information, please reference to kubernetes documentation [Define a liveness HTTP request](https://kubernetes.io/docs/tasks/configure-pod-container/configure-liveness-readiness-startup-probes/#define-a-liveness-http-request)