Mojang/Minecraft Auth

It'd be cool if we could call out to minecraft and decode minecraft usernames. I'm not sure if this is offically supported, and I can't tell if one is actually supposed to use this, so there's probably some more research needed to see if this is even something that should be done.

Anyways [this outlines an auth flow](https://wiki.vg/Authentication) but it isn't oauth compliant.

首先审查链接的 wiki.vg Authentication flow，以及现有的用于关联外部账户的 Flask 端点。验证 Mojang 是否官方支持所提议的用户名解码方式，以及该流程是否能够适配项目的账户关联模型；完成的标准是形成有文档记录的决定，并且在获批的情况下，定义明确的实现范围。

由索引模型根据 Issue 内容生成。