feat(coderd): add webpush package (#17091)

* Adds `codersdk.ExperimentWebPush` (`web-push`)
* Adds a `coderd/webpush` package that allows sending native push
notifications via `github.com/SherClockHolmes/webpush-go`
* Adds database tables to store push notification subscriptions.
* Adds an API endpoint that allows users to subscribe/unsubscribe, and
send a test notification (404 without experiment, excluded from API docs)
* Adds server CLI command to regenerate VAPID keys (note: regenerating
the VAPID keypair requires deleting all existing subscriptions)

---------

Co-authored-by: Kyle Carberry <kyle@carberry.com>

cli/testdata/coder_server_--help.golden

@@ -6,12 +6,12 @@ USAGE:
   Start a Coder server
 
 SUBCOMMANDS:
-    create-admin-user         Create a new admin user with the given username,
-                              email and password and adds it to every
-                              organization.
-    postgres-builtin-serve    Run the built-in PostgreSQL deployment.
-    postgres-builtin-url      Output the connection URL for the built-in
-                              PostgreSQL deployment.
+    create-admin-user           Create a new admin user with the given username,
+                                email and password and adds it to every
+                                organization.
+    postgres-builtin-serve      Run the built-in PostgreSQL deployment.
+    postgres-builtin-url        Output the connection URL for the built-in
+                                PostgreSQL deployment.
 
 OPTIONS:
       --allow-workspace-renames bool, $CODER_ALLOW_WORKSPACE_RENAMES (default: false)