Add Helix API endpoints for anonymous users

[RAnders00]

Anonymous users of Chatterino will not be able to use certain features of Chatterino, if the underlying API is migrated to the Twitch Helix API: Chatterino/chatterino2#1915

Chatterino currently uses these methods: https://github.com/Chatterino/chatterino2/tree/master/src/providers/twitch/api#helix - "get users", "get user follows", "get streams".
And if Chatterino/chatterino2#1928 is implemented, also "get channel" and "get cheeremotes"

The API simply has to provide a forwarder, allowing anybody to make anonymous requests to fetch from these endpoints. The request is then forwarded to Twitch, with an App Access Token used for authentication to be able to access the API.

The modification can then be applied here: https://github.com/Chatterino/chatterino2/blob/c7a2f4cc929681662e3ff0cf7e8d8e2f872039cf/src/providers/twitch/api/Helix.cpp#L335