Introduce fly-castle, a gui relay for flycat

tonight I just release the very first rough version of fly-castle, it is a very simple desktop app to provide a GUI for your own private backup relay written in Rust. the core relay implementation is forked from scsibug/nostr-rs-relay, I might touch the code there to experiment some ideas later.

Currently, I only build for macOS since that's the only OS on my laptop, but maybe I will find a windows computer to build windows one.

Here is how I would love to use it:

macOS

1. download the software from here
2. unzip it, you will get a file named fly-castle
3. create a folder named my-relay and put fly-castle into the folder
4. Start the relay by running the following command on your terminal:

cd my-relay
    sudo chmod 755 fly-castle
    ./fly-castle
    

5. Open flycat website, click the private backup menu on the left side of the page. add the ws://localhost:8080 to your relay via the right side of the new page.

6. if you want to stop running your little relay, just close the window of fly-castle, it is that simple.