feat: add DataFrame.writeJson with JsonWriteOptions #39

Is your feature request related to a problem or challenge?

DataFrame.writeParquet shipped in #27 and a CSV writer is filed
alongside this. JSON is the third writer DataFusion's DataFrame API
exposes natively (DataFrame::write_json), and is the easiest format
to consume from non-Arrow downstream tooling.

Describe the solution you'd like

• Add a JsonWriteOptions value class (single-file output, partition
  columns, compression).
• Add proto/json_write_options.proto, following the convention
  established in feat(write): add DataFrame.writeParquet with ParquetWriteOptions #27.
• Expose DataFrame.writeJson(path[, options]).
• Cover with tests mirroring DataFrameWriteParquetTest.

Describe alternatives you've considered

COPY … TO … via SQL.

Additional context

In the default DataFusion feature set; no Cargo flag changes required.