Support update partial fields

[hudi-bot]

Now Hudi only support update records with all fields present, however, sometimes we may only want to update partial fields.

JIRA info

• Link: https://issues.apache.org/jira/browse/HUDI-1110
• Type: New Feature

[yihua]

We support partial update encoding and partial updates in Spark Merge INTO as well in release 1.0. Closing this.