Set appname and add prefix for SQL Server session (#159)

* Set the JDBC application name to "Spark MSSQL Connector" by default

The change also respects the case when the user passes in applicationName in the `options` Map.

* add prefix for sql server session app name

* modify the form of app name

Co-authored-by: Arvind Shyamsundar <arvindsh@apache.org>

src/main/scala/com/microsoft/sqlserver/jdbc/spark/DefaultSource.scala

@@ -52,7 +52,10 @@ class DefaultSource extends JdbcRelationProvider with Logging {
                     mode: SaveMode,
                     parameters: Map[String, String],
                     rawDf: DataFrame): BaseRelation = {
-        val options = new SQLServerBulkJdbcOptions(parameters)
+        // set SQL Server session application name to SparkMSSQLConnector:user input name
+        // if no user input app name provided, will use SparkMSSQLConnector:NotSpecified
+        val applicationName = s"SparkMSSQLConnector:${parameters.getOrElse("applicationname", "NotSpecified")}"
+        val options = new SQLServerBulkJdbcOptions(parameters  + ("applicationname" -> applicationName))
         val conn = createConnectionFactory(options)()
         val df = repartitionDataFrame(rawDf, options)