api: generate help list per site

[ZenithalHourlyRate]

mirrorz-help has a bunch of helps but not all mirror sites need them, we need to generate a list per site via some API, e.g. https://mirrors.help/api/list/SJTUG-Siyuan

And the result would be a site local name to mirrorz-cname

{
  "site-name": "cname",
  "mx-linux": "mxlinux"
}

Also the list should consider the metadata provided in #58 (comment)

[ZenithalHourlyRate]

Currently, such list is maintained individually, e.g.

• ISCAS : https://mirror.iscas.ac.cn/
• TUNA : https://github.com/tuna/mirrorz-help/blob/0950315c9d2ca6e0e45ca8f1616f85cae76c028d/custom/tuna/transpile.sh#L178
• SJTUG : https://github.com/sjtug/sjtug-mirror-frontend/blob/591f87de77d0e1e65c7736dfdd38100eb5cdadea/src/Data.js#L32