[Issue 453] Add NewMessageID() method.

[crossoverJie]

Fixes #453

Motivation

Sometimes we need to deal with some messages manually.

pb "github.com/apache/pulsar-client-go/pulsar/internal/pulsar_proto"

	msgID := &pb.MessageIdData{
		LedgerId:             proto.Uint64(699),
		EntryId:              proto.Uint64(11),
		Partition:            proto.Int32(0),
		BatchIndex:           proto.Int32(0),
	}
	bytes, _ := proto.Marshal(msgID)
	id, _ := pulsar.DeserializeMessageID(bytes)
	err = consumer.AckID(id)

Here is the nonsensical serialization and deserialization code, also pb is an internal package and cannot be accessed externally.

Modifications

Add the NewMessageID() method.

Verifying this change

• Make sure that the change passes the CI checks.