Configurable Receiver Middleware

[tsutomi]

The WebhookReceiverMiddleware design has been changed radically to support configurations and extensions

• The WebhookReceiverMiddleware is now public and extensible (eg. to implement service-specific middleware, if needed)
• The middleware is not anymore factory-based but rather convention-based: it allows specifying configuration options for the single instance, that can be extended by implementations of service-specific options
• The options to handle the webhooks through the receiver middleware have been exported to another class, specific for the receiving operations.