SSH key support

[natrem]

The rule is working fine but I'd like to use it also with an identity file.
Would you mind a PR on this topic?

The aim is to support authentication via an SSH public/private key pair.

If it's ok for you then there is already an implementation in #15

[Assafkalfa]

I know it's a little old (and using junit4) but I find a way to use a public key:

download the source (only one file) from the git and add to 'startServer' after 'setPort':

SimpleGeneratorHostKeyProvider keyPairProvider = hostKeyPath != null ? new SimpleGeneratorHostKeyProvider(hostKeyPath)
                : new SimpleGeneratorHostKeyProvider();
server.setKeyPairProvider(keyPairProvider);
server.setPublickeyAuthenticator(new AuthorizedKeysAuthenticator(new File(hostKeyPath.toString())));

of course, add private Path hostKeyPath = null in the variables and a setter:

public FakeSftpServerRule setHostKeyPath(Path hostKeyPath) {
       this.hostKeyPath = hostKeyPath;
       if (server != null)
           restartServer();
       return this;
}

[natrem]

@Assafkalfa I do not remember how the code under test was working but the test was implemented quite simply (based on the code in PR #15 :

	@Rule
	public final FakeSftpServerRule sftpServer = new FakeSftpServerRule();

	@Test
	public void should_send_file() throws Exception {
		// add identity
	        Path authorizedKeysPath = Paths.get(FtrServiceTest.class.getResource("/unit-test-user.pub").toURI());
	        sftpServer.addIdentity("jon.snow", authorizedKeysPath);
		
		// call code under test

		//check
		assertTrue("remote file not found", sftpServer.existsFile("/666_myfile.txt"));
	}

It was a one time usage so I cannot help you on other usage of this component.