Added fallback logic to decryption in generated getters

[FlorisTurkenburg]

@brand

In order to be able to roll-over the encryption keys while being able to decrypt values that were encrypted with the old key or with the new key.

Composer.json would then require

'accessor-generator' => [
    'database.table.column' => [
        'public-key'  => 'public',
        'private-key' => 'secret',
     ],
    'database.table.column_fallback' => [
        'private-key' => 'fallback-secret',
     ],
],

See test/Generator/fixtures/expected/CredentialsMethodsTrait.php for how it will be generated