Add mechanism to learn about supported SFTP extensions after connect

After connecting to an SFTP server, the sftp module learns about which SSH extensions are supported. It would be nice if there was a clean way that calling code could learn which of these extensions (`posix-rename@openssh.com`, `statvfs@openssh.com`, `fstatvfs@openssh.com`, `hardlink@openssh.com`, `fsync@openssh.com`, etc) are supported.