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.

cli/testdata/coder_notifications_--help.golden

@@ -19,10 +19,17 @@ USAGE:
     - Resume Coder notifications:
   
        $ coder notifications resume
+  
+    - Send a test notification. Administrators can use this to verify the
+  notification
+  target settings.:
+  
+       $ coder notifications test
 
 SUBCOMMANDS:
     pause     Pause notifications
     resume    Resume notifications
+    test      Send a test notification
 
 ———
 Run `coder --help` for a list of global options.

cli/testdata/coder_notifications_test_--help.golden

@@ -0,0 +1,9 @@
+coder v0.0.0-devel
+
+USAGE:
+  coder notifications test
+
+  Send a test notification
+
+———
+Run `coder --help` for a list of global options.