Allowing model providers to have oauth implementation

[ChaiEranki]

This change allows a model provider to use OAuth similar to what ChatGPT default auth uses. To do this, the following changes are made:

• Extra variables are added to the ModelProviderInfo struct in model_provider_info.rs that allow the user to configure oauth
• A new login screen specifically for custom model provider oauth is created: making it so that if the model provider is configured to use custom oauth, the user will see a set of screens in the beginning showing the status of logging in: codex-rs/tui/src/onboarding/auth.rs
• A new field was added to the auth.json file that stores user tokens telling the user which provider oauth was used to generate it
• New fields were added to the ServerOpts to customize the codex server that is created when oauth is run. This removes hard-coded variables for ChatGPT oauth
• A new AuthMode called ProviderOAuth was added in additon to ChatGPT and ApiKey that already exist

Enhancement request: openai#8937