added oed test file generator

[benhayes21]

Add OED test data generator CLI command

Add a new ods_tools generate CLI command that produces synthetic OED test data files
Loc, Acc, ReinsInfo, ReinsScope) from a JSON configuration. The generator creates files
with referential integrity across all four OED file types and supports configurable
portfolio structure, field selection, financial terms, and output format (csv/parquet).

Changes:

• ods_tools/oed/oed_generator.py — New module containing OEDFileGenerator and
  DataGenerator classes that produce synthetic OED data driven by a JSON config. Schema
  loading updated to use the existing OedSchema class.
• ods_tools/main.py — Registered the generate subcommand with options: --config,
  --output-dir, --format, --example-config, --list-versions, --list-fields, --oed-version.
• tests/data/oed_generator_config.json — Example configuration file for the generator.
• README.md — Documented all CLI commands (convert, check, transform, combine, generate).
  Previously only convert and transform were documented.

Usage:
ods_tools generate --config config.json --output-dir ./output
ods_tools generate --example-config > my_config.json
ods_tools generate --list-fields Loc --oed-version 4.0.0

[benhayes21]

closes #239