INT-3200: DefaultSFTPSessionFactory supports using a ByteArrayResource f...

DefaultSFTPSessionFactory supports using a ByteArrayResource for the PrivateKey
I'd say LGTM, but ti looks for me like we can squesh this new feature with that existing `testPubPrivKey`.
I don't think that there is a reason to add something to the Doc, but we should revise what we have in the `sftp.xml` on the matter.
@liujiong1982 , would you mind to take a look into Docs? It's a part of our work, too.
@garyrussell , any objections to this PR?