add userdata attribut to function SaveCapture

[irov]

using the function is inconvenient with the current implementation. As I need to use globals for it to work

please add

typedef void(*CaptureSaveChunkCb)(const char*,size_t, void * _ud);
OPTICK_API bool SaveCapture(CaptureSaveChunkCb dataCb, void * _ud, bool force = true);