INT-2120:RemoteFileTemplate - Phase III

JIRA: https://jira.spring.io/browse/INT-3205

* add SftpSessionFactoryResolver, DefaultSessionFactoryResolver, SftpSessionFactoryResolver
* add constructor in RemoteFileTemplate that take SessionFactoryResolver as parameter
* add test
Should remain `final`, the same for the `SessionFactoryResolver`: or one of another. Of course, the opposite one should be `null` from consturctors.