Provisioning: Add API endpoint to reload provisioning configs (#16579)

* Add api to reaload provisioning

* Refactor and simplify the polling code

* Add test for the provisioning service

* Fix provider initialization and move some code to file reader

* Simplify the code and move initialization

* Remove unused code

* Update comment

* Add comment

* Change error messages

* Add DashboardProvisionerFactory type

* Update imports

* Use new assert lib

* Use mutext for synchronizing the reloading

* Fix typo

Co-Authored-By: aocenas <mr.ocenas@gmail.com>

* Add docs about the new api

docs/sources/http_api/admin.md

@@ -447,3 +447,36 @@ Content-Type: application/json
   "message": "User auth token revoked"
 }
 ```
+
+## Reload provisioning configurations
+
+`POST /api/admin/provisioning/dashboards/reload`
+
+`POST /api/admin/provisioning/datasources/reload`
+
+`POST /api/admin/provisioning/notifications/reload`
+
+Reloads the provisioning config files for specified type and provision entities again. It won't return
+until the new provisioned entities are already stored in the database. In case of dashboards, it will stop
+polling for changes in dashboard files and then restart it with new configs after returning. 
+
+Only works with Basic Authentication (username and password). See [introduction](http://docs.grafana.org/http_api/admin/#admin-api) for an explanation.
+
+**Example Request**:
+
+```http
+POST /api/admin/provisioning/dashboards/reload HTTP/1.1
+Accept: application/json
+Content-Type: application/json
+```
+
+**Example Response**:
+
+```http
+HTTP/1.1 200
+Content-Type: application/json
+
+{
+  "message": "Dashboards config reloaded"
+}
+```