Feture: Reduce memory allocation during the Encode process to improve encoding performance.

[ethan256]

The current encoding process generates a large amount of memory allocation, and the GC pressure will increase when the QPS becomes large. Consider reducing the number of memory allocations to optimize the encoding process.

Here is my proposal:

1. Modify the BinaryEncoderinterface definition

   type BinaryEncoder interface {
   Encode(s *Stream)
   }

   Reduce NewBuffercalls by stitching each field encoding result directly through Stream. the Streamdefinition is as follows:

   type Stream struct {
   buf []byte
   byte []byte
   err error
   }

   Streamimplements the WriteXxxmethod from the original Buffer.

2. The Encodemethods of all types that implement the BinaryEncoderinterface require only minor modifications, such as DataValue:

   func (d *DataValue) Encode(s *Stream) {
   s.WriteUint8(d.EncodingMask)
   
   if d.Has(DataValueValue) {
       s.WriteAny(d.Value)
   }
   if d.Has(DataValueStatusCode) {
       s.WriteUint32(uint32(d.Status))
   }
   if d.Has(DataValueSourceTimestamp) {
       s.WriteTime(d.SourceTimestamp)
   }
   if d.Has(DataValueSourcePicoseconds) {
       s.WriteUint16(d.SourcePicoseconds)
   }
   if d.Has(DataValueServerTimestamp) {
       s.WriteTime(d.ServerTimestamp)
   }
   if d.Has(DataValueServerPicoseconds) {
       s.WriteUint16(d.ServerPicoseconds)
   }
   }

For detailed implementation, please refer to https://github.com/gopcua/opcua/pull/726