feature #22234 [DI] Introducing autoconfigure: automatic _instanceof configuration (weaverryan)
feature #22234 [DI] Introducing autoconfigure: automatic _instanceof configuration (weaverryan)

This PR was squashed before being merged into the 3.3-dev branch (closes #22234).

Discussion
----------

[DI] Introducing autoconfigure: automatic _instanceof configuration

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | no
| New feature?  | yes (mostly, a continuation of a new feature)
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | https://github.com/symfony/symfony-docs/issues/7538

This is a proposal to allow the user to opt into some automatic `_instanceof` config. Suppose I want to auto-tag all of my voters and event subscribers

```yml
# current
services:
    _defaults:
        autowire: true

    _instanceof:
        Symfony\Component\Security\Core\Authorization\Voter\VoterInterface:
            tags: [security.voter]

        Symfony\Component\EventDispatcher\EventSubscriberInterface:
            tags: [kernel.event_subscriber]

    # services using the above tags
    AppBundle\Security\PostVoter: ~
    AppBundle\EventListener\CheckRequirementsSubscriber: ~
```

If I'm registering a service with a class that implements `VoterInterface`, when would I ever *not* want that to be tagged with `security.voter`? Here's the proposed code:

```yml
# proposed
services:
    _defaults:
        autowire: true
        autoconfigure: true

    # services using the auto_configure_instanceof functionality
    AppBundle\Security\PostVoter: ~
    AppBundle\EventListener\CheckRequirementsSubscriber: ~
```

The user must opt into this and it only applies locally to this configuration file. It works because each enabled bundle would have the opportunity to add one or more "automatic instanceof" definitions - e.g. SecurityBundle would add the `security.voter` instanceof config, FrameworkBundle would add the `kernel.event_subscriber` instanceof config, etc.

For another example, you can check out the proposed changes to `symfony-demo` - symfony/symfony-demo#483 - the `_instanceof` section is pretty heavy: https://github.com/hhamon/symfony-demo/blob/81694ac21e63bebe7ecfa22fa689766f2f38ccd5/app/config/services.yml#L20

Thanks!

Commits
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18627bf9f6 [DI] Introducing autoconfigure: automatic _instanceof configuration

symfony/security-bundle