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/migrations/000312_webpush_subscriptions.down.sql

@@ -0,0 +1,2 @@
+DROP TABLE IF EXISTS webpush_subscriptions;
+

coderd/database/migrations/000312_webpush_subscriptions.up.sql

@@ -0,0 +1,13 @@
+-- webpush_subscriptions is a table that stores push notification
+-- subscriptions for users. These are acquired via the Push API in the browser.
+CREATE TABLE IF NOT EXISTS webpush_subscriptions (
+    id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    user_id UUID NOT NULL REFERENCES users ON DELETE CASCADE,
+    created_at TIMESTAMP WITH TIME ZONE NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    -- endpoint is called by coderd to send a push notification to the user.
+    endpoint TEXT NOT NULL,
+    -- endpoint_p256dh_key is the public key for the endpoint.
+    endpoint_p256dh_key TEXT NOT NULL,
+    -- endpoint_auth_key is the authentication key for the endpoint.
+    endpoint_auth_key TEXT NOT NULL
+);

coderd/database/migrations/testdata/fixtures/000312_webpush_subscriptions.up.sql

@@ -0,0 +1,2 @@
+-- VAPID keys lited from coderd/notifications_test.go.
+INSERT INTO webpush_subscriptions (id, user_id, created_at, endpoint, endpoint_p256dh_key, endpoint_auth_key) VALUES (gen_random_uuid(), (SELECT id FROM users LIMIT 1), NOW(), 'https://example.com', 'BNNL5ZaTfK81qhXOx23+wewhigUeFb632jN6LvRWCFH1ubQr77FE/9qV1FuojuRmHP42zmf34rXgW80OvUVDgTk=', 'zqbxT6JKstKSY9JKibZLSQ==');