Find also custom user named private keys in ~/.ssh

SSHJ by default finds only private keys named:
- id_rsa
- id_dsa
- id_ed25519
- id_ecdsa

It would be nice if it would also pick up user named private keys. This allows us to share code.

I wrote the following code (sorry for the Kotlin):
```
private fun authWithSSHKey(username: String, sshClient: SSHClient)
{
val userSSHConfigFolder = Paths.get(System.getProperty("user.home")).resolve(".ssh")

val list = Files.list(userSSHConfigFolder)

val privateKeyFiles = arrayListOf()
for (path in list)
{
if (Files.isRegularFile(path)
&& path.fileName.toString() != "config"
&& path.fileName.toString() != "known_hosts"
&& !path.fileName.toString().endsWith(".pub"))
{
val absoluteNormalizedString = path.toAbsolutePath().normalize().toString()
privateKeyFiles.add(absoluteNormalizedString)
}
}

sshClient.authPublickey(username, *privateKeyFiles.toTypedArray())
}
```