Implement player certificate fetching for Mojang auth

[evan-goode]

"Mojang auth" (now ironically named) is still in use today by custom authentication servers. We are grateful for node-minecraft-protocol's continued support for it! Some of these custom authentication servers [0][1] support player certificates just like Mojang's API servers do, at the $servicesServer/player/certificates endpoint (when using Mojang's servers, $servicesServer = https://api.minecraftservices.com). The process for fetching player certificates is no different when using a Microsoft account or a Mojang/other account. It works like any other Mojang API route that requires an accessToken. But in node-minecraft-protocol, Microsoft authentication, including certificate fetching, is done in prismarine-auth and certificate fetching isn't implemented for third-party accounts.

This PR copies the certificate fetching logic from prismarine-auth to mojangAuth.js in node-minecraft-protocol to make certificate fetching work with third-party accounts. Certificate fetching can be disabled by setting disableChatSigning: true just like with Microsoft accounts. If certificates fetching fails, a warning is printed and login continues.

An alternative approach to copying would be to export fetchCertificates from prismarine-auth and somehow make it possible to override the servicesServer there. But IMHO the cleaner option is to duplicate the code in node-minecraft-protocol, it's only ~20 lines.

[0] https://github.com/unmojang/drasl
[1] https://ely.by/

[evan-goode]

Thanks for the approval. The CI failure seems to be transient and unrelated.