feat: expose JSON reader via registerJson and readJson

[andygrove]

Is your feature request related to a problem or challenge?

DataFusion 53.1 has built-in support for newline-delimited JSON
(SessionContext::read_json / register_json), but the Java binding
currently only exposes Parquet (#18) and CSV (#21). Users wanting to
query JSON files have to fall back to CREATE EXTERNAL TABLE via SQL,
which loses the typed-options ergonomics the Parquet/CSV bindings
already provide.

Describe the solution you'd like

Mirror the existing reader pattern:

• Add an NdJsonReadOptions value class analogous to
  ParquetReadOptions / CsvReadOptions (file extension, schema,
  schema-infer-max-records, compression, etc.).
• Add a proto/json_read_options.proto and pass options through the
  established proto-over-JNI convention (refactor: pass csv and parquet read options via protobuf #29).
• Expose SessionContext.registerJson(name, path[, options]) and
  readJson(path[, options]).
• Cover with tests in the spirit of SessionContextCsvTest /
  ParquetReadOptionsTest.

Describe alternatives you've considered

Users can issue CREATE EXTERNAL TABLE … STORED AS JSON via
SessionContext.sql. This works but bypasses the typed builder pattern
and gives a less discoverable Java API.

Additional context

DataFusion's JSON reader is in the default feature set, so no Cargo
feature flag changes are required on the native side.