Make cache()->get() return a classed CacheItem object

Problem/Motivation

We return an stdClass from cache->get(), but there's no enforcement of which keys are in it (id? created? expires? tags?), only $cached->data is in everything since code will immediately break without it. To be able to consistently chain cache tag implementations, we'd need to be able to know the tags of a cache item when it's retrieved, something that's not currently required or supported by cache backends.

We can leave $cached->data as a public property at the moment for bc, then remove it in a follow-up since that will touch a lot of code.

Proposed resolution

• Return CacheItem on every cache backend in core
• Wrap cache backends to ensure they return CacheItem always
• Allow cache backends to opt out of this decoration

Remaining tasks

• Write a change record
• Write a Unit test for CacheItemCacheBackendDecorator
• Maybe extend \Drupal\KernelTests\Core\Cache\GenericCacheBackendUnitTestBase?

User interface changes

API changes

Data model changes