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/queries/users.sql

@@ -57,7 +57,7 @@ SELECT
 FROM
 	users
 WHERE
-	(LOWER(username) = LOWER(@username) OR LOWER(email) = LOWER(@email)) AND
+	(LOWER(username) = LOWER(@username) OR (@email != '' AND LOWER(email) = LOWER(@email))) AND
 	deleted = false
 LIMIT
 	1;
@@ -92,13 +92,15 @@ INSERT INTO
 		updated_at,
 		rbac_roles,
 		login_type,
-		status
+		status,
+		is_service_account
 	)
 VALUES
 	($1, $2, $3, $4, $5, $6, $7, $8, $9,
 		-- if the status passed in is empty, fallback to dormant, which is what
 		-- we were doing before.
-		COALESCE(NULLIF(@status::text, '')::user_status, 'dormant'::user_status)
+		COALESCE(NULLIF(@status::text, '')::user_status, 'dormant'::user_status),
+		@is_service_account::bool
 	) RETURNING *;
 
 -- name: UpdateUserProfile :one