…ing cached values in a worker (nicolas-grekas)

This PR was merged into the 5.2-dev branch.

Discussion
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[Cache] add integration with Messenger to allow computing cached values in a worker

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

~~This PR needs and for now embeds #30334. See 2nd commit.~~

Using the new `CacheInterface` enables probabilistic early expiration by default. This means that from time to time, items are elected as early-expired while they are still fresh ([see Wikipedia](https://en.wikipedia.org/wiki/Cache_stampede#Probabilistic_early_expiration) for details).

This PR adds a new `early_expiration_message_bus` option when configuring cache pools. When this option is set, cache pools are configured to send those early-expired items on a Messenger bus, then serve the current value immediately, while the updated value is computed in a worker in the background.

`CacheInterface::get($key, $callback)` accepts any callable, but sending any callable on a bus is not possible (e.g. a `Closure` cannot be serialized). To bypass this constraint, this feature works only with callables in the form `[$service, 'publicMethod']`, where `$service` is any public or [reversible service](symfony/symfony#30334).

This constraint is a serious one: this $service must compute a value when knowing only its key. This means keys should embed enough information for this to happen. I think that's not that hard - and we may find ways to provide additional context in the future.

At least the goal is worth it: in theory, this strategy allows achieving a 100% hit ratio even when invalidation-by-expiration is used.

There are two things one needs to do to enable this behavior:

1. bind a message bus to a cache pool:
```yaml
framework:
    cache:
        pools:
            test.cache:
                early_expiration_message_bus: messenger.default_bus
```

2. route EarlyExpirationMessage to a transport:
```yaml
framework:
    messenger:
        routing:
            'Symfony\Component\Cache\Messenger\EarlyExpirationMessage': amqp
```

Commits
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6c0911f58c [Cache] add integration with Messenger to allow computing cached values in a worker