Add write method to JsonWriter

[matthewmturner]

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I'm looking to add write_json functionality in DataFusion as part of apache/datafusion#1777. Right now write_csv and write_parquet use a writer.write(batch) approach for writing a stream of batches. The current JsonWriter only has write_batches which takes a slice of RecordBatches.

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I would like to add a write method to the writer which takes only a single RecordBatch - similar to the other writers.

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[matthewmturner]

I am going to work on this