Add support for icon URL in NIP-11 document

[0xtrr]

Hi!

I'd love to see support for setting a url for a relay icon in the NIP-11 document, just like it's documented here. I'd love to contribute to this myself but I haven't written a single line of C++ code ever before. I'll try to contribute to this but I post this issue in hopes that someone else with actual C++ experience beat me to it.