Add argument to emscripten_async_wget (and similar wget functions) to set fetch settings

Currently, emscripten_async_wget and its sibling functions emscripten_async_wget_data, emscripten_async_wget2 and emscripten_async_wget2_data do not have a parameter that dynamically sets fetchSettings. Instead fetch settings are either pulled from a Module['fetchSettings'] global constant or, if that is not provided, a default is used.

In at least one fetchSettings case (credentials), it can be necessary to use a different value for different URLs. If credentials = include, that causes CORS errors fetching from cross domains that don't support cross domain credentials. If credentials = same-origin, then credentials won't be shared with cross domains that do support cross domain credentials.

One workaround is to mutate Module['fetchSettings'] just prior to calling emscriptent_async_wget and then reset it just after to get the same effect. However, this is less ergonomic and obvious than a fetchSettings parameter would be.