[SDK-61] Allow users to abort loading a package

[Michael-F-Bryan]

In #378, it'd be nice of we can abort the loading of a package if the arguments to useWasmerPackage() change.

To do this, we need to update things in two places.

First, we need to update the fetch() function in wasmer_wasix::http::web_http_client to use a RAII guard which triggers an abort controller on Drop and pass that abort controller's AbortSignal through to the request (the RequestInit type has a signal setter). That'll make sure any in-progress HTTP requests get cancelled automatically when the future is dropped.

We also need to update wasmer_js::Wasmer's from_registry() and from_file() constructors to accept an optional AbortSignal parameter and use futures::select!() to wait on either the package loading or an abort, whichever resolves first. If the futures::select!() detects an abort, it should automatically drop the future from BinaryPackage::from_registry() or BinaryPackage::from_webc(), which will automatically cancel any child futures until eventually the HTTP request is cancelled.

_SDK-61