Add Twitch Auth

As the marketplace for Twitch grows and solutions like Truffle.dev become realized there is more of a need for authentication with Twitch. There are currently 140 million monthly active Twitch users and many apps catered to the platform require Twitch auth. This would be a great feature to add, and Twitch has good support for auth.

The issue names no files, tests, or entry points. Begin by locating the repository's existing authentication flow and reviewing Twitch's supported auth requirements; define the integration's scope and acceptance checks before implementation.

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