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/modelqueries.go

@@ -480,6 +480,7 @@ func (q *sqlQuerier) GetAuthorizedUsers(ctx context.Context, arg GetUsersParams,
 			&i.HashedOneTimePasscode,
 			&i.OneTimePasscodeExpiresAt,
 			&i.IsSystem,
+			&i.IsServiceAccount,
 			&i.Count,
 		); err != nil {
 			return nil, err