feat: Add C bindings for FDv2 data system

[beekld]

Summary

Exposes the FDv2 data system builder surface in the C API. Adds:

• LDServerFDv2Builder with _Default() and _Custom() factories.
• LDServerFDv2StreamingBuilder and LDServerFDv2PollingBuilder sub-builders, each with BaseURL overrides for per-source endpoint customization.
• Methods to add initializers, synchronizers, and FDv1 fallback sources to an FDv2 builder, plus DisableFDv1Fallback.
• Orchestrator FallbackTimeoutMs and RecoveryTimeoutMs setters.
• LDServerConfigBuilder_DataSystem_FDv2, which installs FDv2 as the SDK's active data system.

Design decisions worth flagging:

• Construction uses _Default() and _Custom() rather than the usual _New(). This mirrors the C++ FDv2Builder's private no-arg ctor + public static factories. Every other C builder in this repo uses _New(); this one deviates deliberately.
• The FDv1 fallback methods accept the existing LDServerDataSourceStreamBuilder / LDServerDataSourcePollBuilder handles. The underlying C++ type is shared between FDv2Builder::FDv1Streaming and BackgroundSyncBuilder::Streaming (both are StreamingBuilder<ServerSDK>), so a dedicated fallback handle would duplicate the surface without adding safety.
• The FDv2 sub-builder types are named LDServerFDv2StreamingBuilder / LDServerFDv2PollingBuilder -- with Streaming / Polling spelled out. The older BackgroundSync C API uses the shorter LDServerDataSourceStreamBuilder / LDServerDataSourcePollBuilder. The new names deliberately mirror the C++ nested type names (FDv2Builder::Streaming / ::Polling).