[HUDI-1716]: Resolving default values for schema from dataframe (#2765)

- Adding default values and setting null as first entry in UNION data types in avro schema. 

Co-authored-by: Aditya Tiwari <aditya.tiwari@flipkart.com>

hudi-spark-datasource/hudi-spark/src/test/resources/exampleEvolvedSchema.txt

@@ -0,0 +1,40 @@
+/*
+  * Licensed to the Apache Software Foundation (ASF) under one
+  * or more contributor license agreements.  See the NOTICE file
+  * distributed with this work for additional information
+  * regarding copyright ownership.  The ASF licenses this file
+  * to you under the Apache License, Version 2.0 (the
+  * "License"); you may not use this file except in compliance
+  * with the License.  You may obtain a copy of the License at
+  *
+  *     http://www.apache.org/licenses/LICENSE-2.0
+  *
+  * Unless required by applicable law or agreed to in writing, software
+  * distributed under the License is distributed on an "AS IS" BASIS,
+  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+  * See the License for the specific language governing permissions and
+  * limitations under the License.
+  */
+ {
+     "namespace": "example.schema",
+     "type": "record",
+     "name": "trip",
+     "fields": [
+         {
+             "name": "_row_key",
+             "type": "string"
+         },
+         {
+             "name": "partition",
+             "type": "string"
+         },
+         {
+             "name": "ts",
+             "type": ["long", "null"]
+         },
+         {
+             "name": "new_field",
+             "type": ["string","null"]
+         }
+     ]
+ }