Formalize a Clippy benchmarking tool + Server

[blyxyas]

While we currently have a way to benchmark Clippy with the official tools (rustc-perf), we don't have:

1. Clippy PR benchmarking

I have a tool for this, becnhv2, but it isn't complete for use yet. It needs to be up and running before hooking it up to the server.

2. Server software for running Rustc-perf with Clippy

It doesn't need to be anything fancy, just some GitHub bot or similar integrated with a server. I've registered an extension called clippy-perf that should serve as a boilerplate. We currently have the server, the only thing left is a way to hook it up to the internet.

It doesn't even have to be written in Rust, as it will just execute a couple commands and return the output as markdown