chore: Add DataSystem contract definition

[jsonbailey]

Note

Introduce DataSystem contract interfaces and related types (availability, diagnostics, result, initializer, synchronizer, update) for the SDK.

• Data System Contracts (lib/ldclient-rb/impl/datasystem.rb):
  • Interfaces:
    • DataSystem mixin with start/stop, status providers, flag_tracker, availability accessors, store, and set_flag_value_eval_fn.
    • Initializer with fetch.
    • Synchronizer with sync yielding Update.
  • Types:
    • DataAvailability with DEFAULTS, CACHED, REFRESHED, and .at_least?.
    • Result with success/failure helpers.
    • Update encapsulating state, change set, error, revert flag, and environment ID.
  • Diagnostics:
    • DiagnosticAccumulator (record stream init, events in batch, create/reset event).
    • DiagnosticSource to inject a diagnostic accumulator.

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