web_backend: Make Client use the PImpl idiom

Like with TelemetryJson, we can make the implementation details private
and avoid the need to expose httplib to external libraries that need to
use the Client class.

src/web_service/telemetry_json.cpp

@@ -4,6 +4,7 @@
 
 #include <json.hpp>
 #include "common/detached_tasks.h"
+#include "common/web_result.h"
 #include "web_service/telemetry_json.h"
 #include "web_service/web_backend.h"