Add Support for the QUOTE Command in FTP Client

[Ning-Qing]

Currently, our FTP client does not support the QUOTE command, which is essential for sending raw, uninterpreted commands directly to the FTP server. Implementing this feature would greatly enhance the flexibility and functionality of our FTP client, allowing users to execute server-specific commands and perform advanced operations.

example:

// Quote send arbitrary ftp command
func (c *ServerConn) Quote(format string, args ...interface{}) (msg string, err error) {
	_, msg, err = c.cmd(2, format, args...)
	return
}

        msg, err := c.Quote("FEAT")
        if err != nil {
	       return 
        }
        fmt.Println(msg)
        // Output:
	// These are my features
	// CLNT
	// UTF8
	// SIZE
	// MDTM
	// REST STREAM
	// EPRT
	// EPSV
	// MLSD
	// MLST
	// MFMT
	// CRC32;MD5;SHA-1;SHA-256*;SHA-512;
	// XCRC
	// MD5
	// XMD5
	// XSHA
	// XSHA1
	// XSHA256
	// XSHA512
        // end