chore: Create FDv2 polling and fallback data source

[jsonbailey]

Note

Introduces the FDv2 polling data path with fallback signaling and supporting infrastructure.

• New Impl::DataSystem::PollingDataSource (Initializer/Synchronizer) with periodic fetch, recoverable/unrecoverable error handling, x-launchdarkly-env-id capture, and FDv1 fallback via x-launchdarkly-fd-fallback
• HTTP requesters: HTTPPollingRequester (FDv2 /sdk/poll) and HTTPFDv1PollingRequester (FDv1 /sdk/latest-all), with ETag handling, selector/filter query params, JSON parsing, and graceful stop
• Payload parsing helpers: polling_payload_to_changeset (FDv2 events) and fdv1_polling_payload_to_changeset (latest-all) producing ChangeSets
• DataSystem factory methods now expose polling_ds_builder and fdv1_fallback_ds_builder; default/streaming/polling configs wire them in
• Result now carries optional headers; ChangeSetBuilder.no_changes uses Selector.no_selector
• Legacy Impl::DataSource::PollingProcessor now stops requestor on shutdown
• Extensive specs added for initializer/synchronizer behavior, header/fallback handling, and payload parsing

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[keelerm84]

Changes seem okay to me, minus the bot feedback.