Add SFTP support to gpfdist for data loading

gpfdist, Cloudberry's parallel file distribution program,
traditionally required data files to be co-located with the gpfdist
process. This limitation made it cumbersome to load data from remote
servers, often requiring an extra data transfer step.

This commit extends gpfdist to support the SFTP protocol, enabling
users to ingest data directly from remote servers. This enhancement
streamlines ETL workflows by allowing `CREATE EXTERNAL TABLE`
to specify SFTP locations.

Key change include:
Fix the compilation errors related to the libssh2 library option
in the configure file.

Author: 王平10304955

src/include/fstream/gfile.h

@@ -72,10 +72,12 @@ typedef struct gfile_t
 #endif
 	} fd;
 
+#ifdef LIBSSH2
 	LIBSSH2_SESSION *session;
 	LIBSSH2_SFTP *sftp_session;
 	LIBSSH2_SFTP_HANDLE *sftp_handle;
 	int sock;
+#endif
 
 	union
 	{