allow overriding HttpResponse.getContent to install download throttler

We need to throttle downloads from cloud storage in order to minimize GC disturbances during (data) reload. I was unable to find a clean way to do this via existing API. Instead, the following workaround was implemented which suffers from extending `HttpURLConnection`. Is it possible to provide a proper hook for intercepting `InputStream`?

```java
protected Storage installHttpThrottler(StorageOptions.Builder storageOptions) {
return storageOptions
.setTransportOptions(HttpTransportOptions.newBuilder().setHttpTransportFactory(new ThrottledHttpTransportFactory()).build())
.build()
.getService();
}
```

```java
class ThrottledHttpTransportFactory implements HttpTransportFactory {
public HttpTransport create() { return new NetHttpTransport.Builder().setConnectionFactory(new ThrottledConnectionFactory()).build(); }
}
```

```java
class ThrottledConnectionFactory extends DefaultConnectionFactory {
@Override
public HttpURLConnection openConnection(URL url) throws IOException {
return new ThrottledUrlConnection(super.openConnection(url), url);
}
}
```

```java
class ThrottledUrlConnection extends HttpURLConnection {
HttpURLConnection delegate;

ThrottledUrlConnection(HttpURLConnection delegate, URL url) {
super(url);
this.delegate = delegate;
}

@Override
public InputStream getInputStream() throws IOException {
return new ThrottledInputStream(delegate.getInputStream(), rateLimiter, throughputMeter);
}
// followed by a long list of overridden methods to redirect via 'delegate'
....
}
```