NIFI-10158 Correct ListFTP expression support for Hostname and Port

[exceptionfactory]

Summary

NIFI-10158 Corrects ListFTP support for expression language in Hostname and Port properties.

The ListFTP processor declares support for expression language values derived from a variable registry, but property resolution did not perform expression evaluation in StandardFTPClientProvider. ListFTP does not have access to a FlowFile and so provides an empty Map of attributes, whereas other FTP processors provide a Map of attributes from an input FlowFile.

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[thenatog]

Tested this with a virtual box FTP server and found that I was able to set variables in my flow and reference these in the ListFTP processor to connect and list files on that server. +1 will merge soon.