I don't know any libraries for this in any good backend languages, but I've worked with these packages in NodeJS to do something like that:
- https://www.npmjs.com/package/http-proxy
- https://www.npmjs.com/package/connect
- https://www.npmjs.com/package/harmon
If you don't want to act like a proxy, you're going to approach this like a normal web applications that does HTTP requests using whatever HTTP client your framework of choice uses.