Changes

card_key_provider: separate column decryption

The concrete class CardKeyProviderCsv supports the decryption of
encrypted CSV columns (fields). However, this kind of mechanmism
may also be useful in with other data formats we may implement in
the future, so let' slplit the encryption part into a seperate
class from which we may inherit.

Related: SYS#7725
Change-Id: I180457d4938f526d227c81020e4e03c6b3a57dab