feat: add FDv2 configuration builder

[beekld]

Stacked on #540. Adds a public FDv2Builder and wires the FDv2 data system into MakeDataSystem, so end users can opt into the FDv2 protocol via the existing config-builder pattern.

// Default FDv2 stack (streaming primary, polling fallback,
// FDv1 streaming last-resort)
auto config = ConfigBuilder("sdk-key-123")
    .DataSystem().Method(DataSystemBuilder::FDv2())
    /* ... */;

// Customised: payload filter + shorter fallback timeout, no FDv1 fallback
auto config = ConfigBuilder("sdk-key-123")
    .DataSystem().Method(
        DataSystemBuilder::FDv2()
            .Streaming(FDv2Builder::StreamingSource().Filter("mobile-flags"))
            .FallbackTimeout(std::chrono::seconds{30})
            .DisableFDv1Fallback())
    /* ... */;

Test plan

• 5 new FDv2 builder tests in config_builder_test.cpp
• Full server suite green (503 tests)