Replace default delivery with HttpClient implementation

[yousif-bugsnag]

Goal

Replace the obsolete HttpWebRequest based delivery with a new default implementation using HttpClient. This allows us to remove the dependency on System.Net.Requests (which has dependencies with known vulnerabilities).

Design

The old ThreadQueueDelivery class has been replaced with a new DefaultDelivery class that uses HttpClient to send requests to the configured endpoints.

The new implementation maintains the queue-based delivery mechanism, but with a few changes/improvements:

• Replaces the hand-cranked BlockingQueue<IPayLoad> implementation with a BlockingCollection<IPayLoad> from .NET, which handles thread-safety internally.
• Uses Task.Run instead of using a dedicated thread that runs continuously.
• Uses a CancellationTokenSource for cleaner shutdown.

The HttpClient is configured to use the proxy from client configuration (if set) and the Proxy property has been removed from the IPayLoad interface - this means that setting/overriding the proxy for individual reports in a BeforeNotify call is no longer supported.

Testing

Added new unit tests and tested manually in a sample project