(S)FTP rotating server advice uses String as key but should use Object

org.springframework.integration.file.remote.aop.RotatingServerAdvice.KeyDirectory uses String as key datatype whereas org.springframework.integration.file.remote.session.DelegatingSessionFactory uses Object as key datatype.

In order to make both APIs logically equal, KeyDirectory should also use Object as key datatype for scenarios where key types other than String are used in conjunction with DelegatingSessionFactory.

Feature of rotating (S)FTP servers for inbound channel adapters was introduced with INT-4491.