chore: Create FDv2 streaming data source

[jsonbailey]

Note

Introduces FDv2 streaming support and integrates it behind an opt-in configuration.

• Adds StreamingDataSourceV2 (SSE-based) with error handling, heartbeats, FDv1 fallback via headers, and diagnostics; includes builder StreamingDataSourceBuilder
• Extends LaunchDarkly::DataSystem with streaming_ds_builder and predefined configs (default, streaming, polling); updates LDClient to use FDv2 when config.data_system_config is set, otherwise FDv1
• Renames config option/accessor to data_system_config and updates references/examples
• Converts FDv2 event names to symbols and aligns consumers (polling payload parsing, protocol docs/returns)
• Increases ld-eventsource runtime dependency to 2.4.0
• Adds unit tests for streaming message handling and error/reset behavior

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