Feature Request: Allowing OAuth similar to ChatGPT for custom providers

What feature would you like to see?

We are part of Oracle Code Assist and we do not use API Key auth for our provider. One solution would be to add config to the model provider setup for oauth settings. Then the user would be able to access other more complicated providers using codex cli. This would not be a huge change since ChatGPT already uses this form of authentication and we would just be extending it.

These are some of the example fields we would add to the ModelProviderInfo struct:

``` rust
// Does the provider use it's own oauth mechanism, if so the following fields are to do
// with configuration of it

// Flag for whether to use custom oauth
pub requires_custom_oauth: bool,

// client id for oauth
pub client_id: Option<String>,

// issuer for oauth
pub issuer: Option<String>,

// refresh_token_path
pub refresh_token_path: Option<String>,

// issuer_path_prefix
pub issuer_path_prefix: Option<String>,

// redirect callback path
pub redirect_callback_path: Option<String>,

// refresh token form data
pub refresh_token_form_data: Option<bool>,

// port for auth server for oauth
pub auth_server_port: Option<u16>,

// refresh token interval in minutes
pub refresh_token_interval_minutes: Option<i64>,

### Additional information

_No response_