add support in the mock-api for auth/token?grant_type=authorization_code

[dafreaki]

Currently only ?grant_type=client_credentials is supported

[lleadbet]

Thinking through this, unfortunately I don't know if it makes sense to support this off the cuff; to support this, we'd need some sort of pseudo-login system as well, including a basic UI.

It's not out of the question, but definitely a backlog. We support getting access tokens via the custom user_token grant, noted here: https://github.com/twitchdev/twitch-cli/blob/main/docs/mock-api.md#auth-namespace