Actually use the async code paths for bulk insert (#93)

Since its introduction in 045033c, the `sync` argument passed to `BulkInsert()` has always been hardcoded to `false` instead of forwarding the `sync` parameter.

As a result, the async code paths for bulk insert in all providers would never be used. This can be confirmed by running code coverage.

Author: 0xced

src/PhenX.EntityFrameworkCore.BulkInsert/BulkInsertProviderBase.cs

@@ -144,7 +144,7 @@ private async Task<string> PerformBulkInsertAsync<T>(
         activity?.AddTag("tempTable", tempTableRequired);
         activity?.AddTag("synchronous", sync);
 
-        await BulkInsert(false, context, tableInfo, entities, tableName, columns, options, ctk);
+        await BulkInsert(sync, context, tableInfo, entities, tableName, columns, options, ctk);
         return tableName;
     }