Major Description
The current DataSourceWriter API makes it hard to implement onTaskCommit(taskCommit: TaskCommitMessage) in FileCommitProtocol.
In general, on receiving commit message, driver can start processing messages(e.g. persist messages into files) before all the messages are collected.
The proposal to add a new API:
add(WriterCommitMessage message): Handles a commit message on receiving from a successful data writer.
This should make the whole API of DataSourceWriter compatible with FileCommitProtocol, and more flexible.
There was another radical attempt in #20386. Creating a new API as #20454 is more reasonable.
