Issue with Custom Encryptor

[cs-akash-jarad]

currently, the key mapping process is considered using Halite and Defuse only if i give a custom call path for my Encrypter class then it expected the key to be at that path .
eg :
at line 37 in src/DependencyInjection/DoctrineEncryptExtension.php
$config['encryptor_class_full'] = $config['encryptor_class'];
and for mapping the key path below code is used
$config['secret_directory_path'].'/.'.$config['encryptor_class'].'.key'
issue is if i give my Encryptor calls Path as App\Encryptors\CustomEncryption
and my secret_directory_path path as /opt/application/keypath/

the formation of the key will be expected /opt/application/keypath/App\Encryptors\CustomEncryption
which is not the case.

my proposed solution is to add a new config param as "secret_key_name" if set then it will make the encryption key path accordingly.

PR: #55