See #334 for more information on the underlying issue.

This PR creates a private URLSessionDelegateWrapper class to act as an intermediary between URLSessionHTTPClient and URLSession.delegate. From the inline documentation I added:

/// This class exists to avoid a retain cycle between `URLSessionHTTPClient` and its underlying
/// `URLSession.delegate`. Since `URLSession` retains its `delegate` strongly, setting the
/// `URLSessionHTTPClient` directly as the `delegate` will cause a retain cycle:
/// https://developer.apple.com/documentation/foundation/urlsession/1411597-init#parameters
///
/// To work around this, `URLSessionDelegateWrapper` maintains a `weak` reference to the
/// `URLSessionHTTPClient` and passes delegate calls through to it, avoiding the retain cycle.

The client now also calls self.session.finishTasksAndInvalidate() on deinit.

This workaround isn't the cleanest, but it does solve the issue and I'm not sure there's a better way of doing it. Open to suggestions, though.

I also made the builder function in the example app static to avoid capturing self in the builder closures.

With these changes, I was able to confirm the cycle demonstrated in the above issue is resolved in the memory debugger:

Note that I avoided changing the open func signatures of URLSessionHTTPClient since that would be a breaking API change.

Resolves #334.