function amf0encLongString(str) {
  let buf = Buffer.alloc(5);
  buf.writeUInt8(0x0C, 0);
  buf.writeUInt32BE(str.length, 1);
  return Buffer.concat([buf, Buffer.from(str, "utf8")]);
}
AMF0 Encode Long String
@param {string} str
@returns {Buffer}