[5.8] Fix PSR-16 TTL conformity

[driesvints]

Resend (and a better implementation) for #27166

These changes to the Cache Repository interface and implementation will make our cache implementation compliant with the PSR-16 standard's TTL requirements. It contains some significant changes in how we handle and store cache values.

The main change that was done is that now a TTL with a value of NULL will be considerd an attempt to store the item forever. In addition to this, any value with a TTL equal or lower than zero will be considerd an attempt to remove the item from the cache. The put, putMany and add methods were updated to reflect these changes.

Before these changes a request like Cache::put('foo', 'bar') wouldn't do anything since a NULL TTL meant that the call was simply ignored. Now any request without a specified TTL will have the default TTL of NULL and thus the request is considered to attempting to store the item forever.

For the putMany call a NULL TTL now means that every single item will be stored forever individually. As there isn't a putManyForever equivalent on the Repository or the PSR interface, there was no way to tell a cache store to store the given items in one go.

For the add call, the behavior to ignore the call when a zero or less TTL was given is kept but when a NULL TTL is now passed, it's correctly passed to the put method. This will ignore a custom add method on the cache store as any NULL TTL simply could be considered to re-store the item indefinitely.

No changes were made to the cache stores themselves. Only the Repository was modified. This way, the cache stores keep their current behavior as they aren't implementations of the PSR Simple Cache interface.

All in all these changes will now make us conform with the PSR spec much better.

For reference: https://www.php-fig.org/psr/psr-16/

Fixes #27160

PS. As a proof of concept I was able to simplify a call in the Filesystem component.

[asgrim]

Had a quick look, this looks good 👍 I like that null is checked both in repository and in each store implementation :)

[driesvints]

Hey @asgrim, thanks for reviewing. Please note that the changes from the other PR (that wasn't merged) aren't in this one. null isn't checked anymore in each store implementation. In this version, null doesn't reaches the individual stores anymore. The repository itself handles them.

[taylorotwell]

Added integration tests for Memcached + tagged cache items.