Add option to turn off second level cache

[jankonas]

Contrary to other caches, second level cache cannot be turn off by passing Doctrine\Common\Cache\VoidCache to it's configuration.

This PR adds option for disabling second level cache by passing false to it's configuration:

nettrine.orm.cache:
  secondLevelCache: false

[codecov]

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Additional details and impacted files

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  Coverage          ?   88.95%           
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  Files             ?       11           
  Lines             ?      344           
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  Hits              ?      306           
  Misses            ?       38           
  Partials          ?        0           

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