* GH-2996: Add Resource for remote file transfer
Fixes https://github.com/spring-projects/spring-integration/issues/2996
* Fix `RemoteFileTemplate.send()` to support a `Resource` payload
for remote file transferring content
* Code style clean up for `RemoteFileTemplate`
* Remove `volatile` for configuration properties for better performance
* Change a `charset` to the `Charset` for only once conversion from string
during configuration phase
* Fix (S)FTP tests for new functionality
* Change affected tests to JUnit 5
* Document a new feature; mention all the supported types and `FileExistsMode` constants
* * Fix language in `whats-new.adoc`
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