chore: Use FDv1 DataSystem in the ldclient

[jsonbailey]

Note

Replaces legacy data-source/store wiring in LDClient with FDv1 DataSystem and updates client to use its store, status providers, and broadcasters.

• DataSystem (impl/data_system/fdv1.rb):
  • Add FDv1 implementation wiring v1 data sources/stores behind a unified interface.
  • Wrap original config.feature_store in FeatureStoreClientWrapper (avoid nested wrapping on postfork) and expose status providers/broadcasters/update sinks.
  • Manage processor lifecycle (start/stop), executor, and compute data availability/target; log when streaming is disabled (polling).
• Core (ldclient.rb):
  • Initialize and use FDv1 for store access (@data_system.store), flag change broadcaster, and data/data-store status providers.
  • Simplify startup/shutdown and postfork by delegating to @data_system; remove legacy data source plumbing.
  • Update initialized? to compare data_availability vs target_availability.
  • Adjust SDK key nil validation to allow LDD or custom data source without events.
  • Update flag evaluation and all_flags_state to read from @data_system.store.
• Tests (spec/impl/data_system/fdv1_spec.rb):
  • Add comprehensive specs for store wrapping behavior, processor selection (streaming/polling/offline/LDD/custom), lifecycle, availability states, and diagnostic accumulator propagation.

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[jsonbailey]

The file used a mix of per method private and all methods after the declaration on line 717 of the original file. Moving to defining per method.