Add key-value-store cache

This adds a CacheProvider class that can be used in Doctrine to use the key-value-store as a cache (metadata, query, result).

In Symfony, you would configure these classes as services:
```services:
    foundationdb_database:
        class: FDB\Database
        factory_class: FDB\KeyValue\DatabaseFactory
        factory_method: openAPI
        arguments:
            - 300
    foundationdb_cache_service:
        class: FDB\KeyValue\Cache\FoundationDBCache
        arguments: [@foundationdb_database, 'evaluations_cache']
```

The Doctrine config would look like this:

```    orm:
        ...
        query_cache_driver:
            type: service
            id: foundationdb_cache_service
        metadata_cache_driver:
            type: service
            id: foundationdb_cache_service
        result_cache_driver:
            type: service
            id: foundationdb_cache_service
```

Would you be interested in adding such a thing? (it works as a PoC, not sure if it's valuable)