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>

coderd/database/dump.sql

@@ -1614,6 +1614,15 @@ CREATE TABLE user_status_changes (
 
 COMMENT ON TABLE user_status_changes IS 'Tracks the history of user status changes';
 
+CREATE TABLE webpush_subscriptions (
+    id uuid DEFAULT gen_random_uuid() NOT NULL,
+    user_id uuid NOT NULL,
+    created_at timestamp with time zone DEFAULT CURRENT_TIMESTAMP NOT NULL,
+    endpoint text NOT NULL,
+    endpoint_p256dh_key text NOT NULL,
+    endpoint_auth_key text NOT NULL
+);
+
 CREATE TABLE workspace_agent_devcontainers (
     id uuid NOT NULL,
     workspace_agent_id uuid NOT NULL,
@@ -2305,6 +2314,9 @@ ALTER TABLE ONLY user_status_changes
 ALTER TABLE ONLY users
     ADD CONSTRAINT users_pkey PRIMARY KEY (id);
 
+ALTER TABLE ONLY webpush_subscriptions
+    ADD CONSTRAINT webpush_subscriptions_pkey PRIMARY KEY (id);
+
 ALTER TABLE ONLY workspace_agent_devcontainers
     ADD CONSTRAINT workspace_agent_devcontainers_pkey PRIMARY KEY (id);
 
@@ -2745,6 +2757,9 @@ ALTER TABLE ONLY user_links
 ALTER TABLE ONLY user_status_changes
     ADD CONSTRAINT user_status_changes_user_id_fkey FOREIGN KEY (user_id) REFERENCES users(id);
 
+ALTER TABLE ONLY webpush_subscriptions
+    ADD CONSTRAINT webpush_subscriptions_user_id_fkey FOREIGN KEY (user_id) REFERENCES users(id) ON DELETE CASCADE;
+
 ALTER TABLE ONLY workspace_agent_devcontainers
     ADD CONSTRAINT workspace_agent_devcontainers_workspace_agent_id_fkey FOREIGN KEY (workspace_agent_id) REFERENCES workspace_agents(id) ON DELETE CASCADE;