Add in Log line numbers for streaming logs (#663)

* Add in Log line

Co-authored-by: yaananth (Yash) <yaananth@github.com>

src/Sdk/DTWebApi/WebApi/TimelineRecordLogLine.cs

@@ -0,0 +1,29 @@
+using System;
+using System.Runtime.Serialization;
+
+namespace GitHub.DistributedTask.WebApi
+{
+    [DataContract]
+    public sealed class TimelineRecordLogLine
+    {
+        public TimelineRecordLogLine(String line, long? lineNumber)
+        {
+            this.Line = line;
+            this.LineNumber = lineNumber;
+        }
+
+        [DataMember]
+        public String Line
+        {
+            get;
+            set;
+        }
+
+        [DataMember (EmitDefaultValue = false)]
+        public long? LineNumber
+        {
+            get;
+            set;
+        }
+    }
+}