Allow user to override raw github sources

As title, access to raw.githubusercontent.com is an issue for Chinese users, and when running bundle exec tapioca init, I got the following:

$ bundle exec tapioca init
   identical  sorbet/config
   identical  sorbet/tapioca/config.yml
   identical  sorbet/tapioca/require.rb
       force  bin/tapioca
Retrieving index from central repository... 
Can't fetch file `index.json` from https://raw.githubusercontent.com/Shopify/rbi-central/main (Failed to open TCP connection to raw.githubusercontent.com:443 (Connection refused - connect(2) for "raw.githubusercontent.com" port 443))

Tapioca can't access the annotations at https://raw.githubusercontent.com/Shopify/rbi-central/main.

Are you trying to access a private repository?
If so, please specify your Github credentials in your ~/.netrc file or by specifying the --auth option.

See https://github.com/Shopify/tapioca#using-a-netrc-file for more details.
Can't fetch annotations without sources (no index fetched)

Start by tracing the source retrieval used by bundle exec tapioca init, especially its access to https://raw.githubusercontent.com/Shopify/rbi-central/main. Determine where an override can be configured and how it should affect fetching index.json and annotations; done means users can select an alternate raw GitHub source and initialization completes without the connection error.