Store jitsu js code on project after initialization

What does this MR do and why?

• In Add interface to create analytics stack for pro... (!95999 - merged) we created an interface by which a GitLab project can initialize the required Jitsu configuration via a service object.
• However, in that MR we didn't pass the generated JS token back to GitLab which will need to be displayed to the user.
• This MR adds a new database column to Project and stores the value there.

How to set up and validate locally

1. Set up the devkit and start it running. docker-compose up

2. In this branch, run rails c to open the rails console

3. Enable the feature flag

   Feature.enable(:cube_api_proxy)

4. Set the required application settings:

   The Jitsu attributes are set up when first configuring your jitsu instance.

   ApplicationSetting.current.update(jitsu_project_xid: 'XXXXXXXXXX', jitsu_host: 'http://localhost:8000', clickhouse_connection_string: 'http://test:test@clickhouse:8123', jitsu_administrator_email: 'XXXXXXX', jitsu_administrator_password: 'XXXXXXX')
   reload!

5. Find a project on the console: project = Project.find(PROJECT_ID)

6. Run ProductAnalytics::InitializeStackService.new(container: project).execute

7. Ensure that a sidekiq job was enqueued.

8. Check the project and make sure that the Jitsu key has been stored: project.project_setting.jitsu_key

9. In the Jitsu configurator, ensure that a new API key (with the project ID) has been created, along with a new associated connection to a clickhouse destination. 👇

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

/related #371206 (closed)