INTSAMPLES-134: SFTP: Use Apache SSHD SshServer

JIRA: https://jira.spring.io/browse/INTSAMPLES-134

INTSAMPLES-134: `sun.misc.BASE64Decoder` -> `org.apache.sshd.common.util.Base64`

INTSAMPLES-134: PR comments and fix for Windows

INTSAMPLES-134 Test Samples with Remote Server

- change 'user' property to 'username' to avoid conflict with environment variable.
- create temporary files using the remote file template in 'si.sftp.sample'.
- clean up after test completion.
- use a caching session factory.