feat: backend support for creating and storing service accounts (#22698)

Add is_service_account column to users table with CHECK constraints
enforcing login_type='none' and empty email for service accounts.
Update user creation API to validate service account constraints.

Related to:
https://linear.app/codercom/issue/PLAT-27/feat-backend-support-for-creating-and-storing-service-accounts

coderd/database/migrations/000299_user_configs.down.sql

@@ -4,10 +4,12 @@ ALTER TABLE users ADD COLUMN IF NOT EXISTS
 
 -- Copy "theme_preference" back to "users"
 UPDATE users
-	SET theme_preference = (SELECT value
-    FROM user_configs
-    WHERE user_configs.user_id = users.id
-		  AND user_configs.key = 'theme_preference');
+	-- Use COALESCE(SELECT, <default>) to avoid forcing an insert of user_configs
+	-- for every users insert in order for this down migration to succeed.
+	SET theme_preference = COALESCE(
+		(SELECT value FROM user_configs WHERE user_configs.user_id = users.id AND user_configs.key = 'theme_preference'),
+		''
+	);
 
 -- Drop the "user_configs" table.
 DROP TABLE user_configs;