feat: add options support to Store for format-specific storage configuration

[james-willis]

Summary

• Adds a new Store.forDownload(StorageFormat format, Map<String, String> options) factory method, allowing users to pass format-specific storage options (e.g. ignoreNullFields, header, delimiter) that correspond to Spark's OPTIONS (...) clause
• The options map is serialized as a nested JSON object in the store field of the execute_sql request
• Existing forDownload() and forDownload(format) methods are fully backward compatible — no changes required for current users

Motivation

Users storing results via wstmt.setStore(Store.forDownload()) had no way to pass configurations like:

CREATE TABLE output_table
USING geojson
OPTIONS (ignoreNullFields 'false')

Now they can:

wstmt.setStore(Store.forDownload(StorageFormat.geojson, Map.of("ignoreNullFields", "false")));

Changes

• Store.java — Added Map<String, String> options field, new forDownload(format, options) factory method, and getOptions() getter. Options are defensively copied and stored as an unmodifiable map; empty/null options are normalized to null and omitted from JSON serialization.
• StoreTest.java (new) — 8 tests covering serialization, backward compatibility, empty/null handling, immutability, and end-to-end ExecuteSqlRequest serialization.
• README.md — Updated Store Options section with the new factory method and usage examples.