Kosuke Saigusa

Update signing logic for correct custom token format

3

## Background Create custom token here: ```dart final adminApp = FirebaseAdminApp.initializeApp( 'project-id', Credential.fromServiceAccountParams( clientId: 'client-id', privateKey: 'private-key', email: 'email', ), ); final auth = Auth(adminApp); final customToken = await auth.createCustomToken('some-user-id');...