feat: add FDv2 configuration builder

[beekld]

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)

---

Note

Medium Risk
Changes how the server SDK selects and connects its data source (new public API and client construction path); default remains Background Sync unless FDv2 is configured.

Overview
Introduces FDv2Builder and FDv2Config so apps can select the FDv2 changeset protocol via DataSystemBuilder::Method(FDv2::Default()) or a custom stack (ordered initializers/synchronizers, optional FDv1 streaming/polling fallback, fallback/recovery timeouts, per-source BaseUrl overrides).

MakeDataSystem in client_impl.cpp now constructs FDv2DataSystem from config: polling initializer factories plus streaming/polling synchronizer factories and optional FDv1 adapter factories, with shared fallback/recovery condition factories.

FDv2 poll/stream implementations take a resolved base URL string (override or service endpoints) instead of full ServiceEndpoints. New initializer/synchronizer factory types build runtime sources from built config.

config_builder_test.cpp adds FDv2 builder coverage; existing FDv2 HTTP tests updated for the new request API.

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