feat(cache): support stale while revalidate

[innocenzi]

This pull request adds support for the "stale while invalidate" pattern on the cache implementation.

$cache->resolve(
    key: 'cache-key',
    callback: fn () => $this->computeExpensiveValue(),
    expiration: Duration::minute(), // should expire in a minute
    stale: Duration::minute() // but allow a stale period of one more minute
);

When requested in the stale window (in this example, from one minute to two minutes after caching the value), a deferred callback will be registered for refreshing the value.