feat: add tool to send a test notification (#16611)

Relates to https://github.com/coder/coder/issues/16463

Adds a CLI command, and API endpoint, to trigger a test notification for
administrators of a deployment.

docs/reference/cli/notifications.md

@@ -26,11 +26,17 @@ server or Webhook not responding).:
   - Resume Coder notifications:
 
      $ coder notifications resume
+
+  - Send a test notification. Administrators can use this to verify the notification
+target settings.:
+
+     $ coder notifications test
 ```
 
 ## Subcommands
 
-| Name                                             | Purpose              |
-|--------------------------------------------------|----------------------|
-| [<code>pause</code>](./notifications_pause.md)   | Pause notifications  |
-| [<code>resume</code>](./notifications_resume.md) | Resume notifications |
+| Name                                             | Purpose                  |
+|--------------------------------------------------|--------------------------|
+| [<code>pause</code>](./notifications_pause.md)   | Pause notifications      |
+| [<code>resume</code>](./notifications_resume.md) | Resume notifications     |
+| [<code>test</code>](./notifications_test.md)     | Send a test notification |

docs/reference/cli/notifications_test.md

@@ -0,0 +1,10 @@
+<!-- DO NOT EDIT | GENERATED CONTENT -->
+# notifications test
+
+Send a test notification
+
+## Usage
+
+```console
+coder notifications test
+```