Expose PGP symmetric encryption algorithm in EncryptContent

EncryptContent, when using PGP, is currently hard coded to use AES_128 as its symmetric key algorithm.  There is a TODO adjacent to this in the code [1]:

     // TODO: Refactor internal symmetric encryption algorithm to be customizable 

[1] https://github.com/apache/nifi/blob/master/nifi-nar-bundles/nifi-standard-bundle/nifi-standard-processors/src/main/java/org/apache/nifi/security/util/crypto/OpenPGPKeyBasedEncryptor.java#L348