Create RESTful API proxy in .NET

Project Desciption

I need software created that will act as a proxy between my application and a third-party API service. I would like the proxy to accept the incoming authenticated request, forward it along to the third-party API, wait for the response and then respond back to the application. 

The proxy must have the architecture to allow me to modify the response back to the application, whether it is before or after the actual service call. For instance, if I want to add 20 seconds to the API call, the proxy should be able to simulate that. Another example would be to send back a response without ever forwarding it along to the third-party service.

If software like this already exists in .NET in a GitHub project, I will award a finders fee, so please message me. 

The first milestone in the project will be to forward a request to a specific third-party API. I will share those details with the winner.

Thanks for reading. Please respond with a non-generic message so I know that you read the entire description.