Make sonata-project/notification-bundle optional

[core23]

Subject

I am targeting this branch, because this feature is BC.

Closes #1238

Changelog

### Changed
- Publishing is synchronous by default
- Made `sonata-project/notification-bundle` optional

To do

• Update the tests;
• Update the documentation;
• Add an upgrade note.

[SonataCI]

Could you please rebase your PR and fix merge conflicts?

[VincentLanglet]

Do you plan/have time to finish this @core23 ? :)

[core23]

I lost interest for this bundle. So I haven't time to finish this :(

[github-actions]

This PR has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.