feat: Add the FDv2 streaming synchronizer to the client SDK

libs/client-sdk/src/CMakeLists.txt

@@ -20,6 +20,7 @@ target_sources(${LIBNAME} PRIVATE
         data_sources/fdv2/fdv2_polling_impl.cpp
         data_sources/fdv2/polling_initializer.cpp
         data_sources/fdv2/polling_synchronizer.cpp
+        data_sources/fdv2/streaming_synchronizer.cpp
         data_sources/data_source_event_handler.cpp
         data_sources/polling_data_source.cpp
         flag_manager/flag_store.cpp
@@ -45,6 +46,7 @@ target_sources(${LIBNAME} PRIVATE
         data_sources/fdv2/fdv2_polling_impl.hpp
         data_sources/fdv2/polling_initializer.hpp
         data_sources/fdv2/polling_synchronizer.hpp
+        data_sources/fdv2/streaming_synchronizer.hpp
         flag_manager/flag_store.hpp
         flag_manager/flag_updater.hpp
         bindings/c/sdk.cpp

libs/client-sdk/src/data_sources/fdv2/fdv2_response_headers.cpp

@@ -32,4 +32,29 @@ FDv2ResponseHeaders ReadFDv2ResponseHeaders(
     return result;
 }
 
+FDv2ResponseHeaders ReadFDv2ResponseHeaders(
+    boost::beast::http::response_header<> const& headers) {
+    FDv2ResponseHeaders result;
+
+    if (auto const it = headers.find(kEnvironmentIdHeader);
+        it != headers.end()) {
+        result.environment_id = std::string{it->value()};
+    }
+
+    auto const fallback = headers.find(kFDv1FallbackHeader);
+    if (fallback == headers.end() ||
+        !boost::iequals(fallback->value(), "true")) {
+        return result;
+    }
+
+    auto const ttl = headers.find(kFDv1FallbackTtlHeader);
+    result.fdv1_fallback =
+        ttl == headers.end()
+            ? FDv1FallbackDirective::DefaultTtl()
+            : FDv1FallbackDirective::FromServiceTtl(
+                  std::string_view{ttl->value().data(), ttl->value().size()});
+
+    return result;
+}
+
 }  // namespace launchdarkly::client_side::data_sources

libs/client-sdk/src/data_sources/fdv2/fdv2_response_headers.hpp

@@ -4,6 +4,8 @@
 
 #include <launchdarkly/network/http_requester.hpp>
 
+#include <boost/beast/http/message.hpp>
+
 #include <optional>
 #include <string>
 
@@ -20,4 +22,7 @@ struct FDv2ResponseHeaders {
 FDv2ResponseHeaders ReadFDv2ResponseHeaders(
     network::HttpResult::HeadersType const& headers);
 
+FDv2ResponseHeaders ReadFDv2ResponseHeaders(
+    boost::beast::http::response_header<> const& headers);
+
 }  // namespace launchdarkly::client_side::data_sources