Function setup 

pub async fn setup(
    rocket: Rocket<Build>,
    config: OIDCConfig,
) -> Result<Rocket<Build>, Box<dyn Error>>

Initializes the Rocket application with OpenID Connect authentication support.

This function:

• Loads OIDC configuration from the given config.
• Calls from_provider_oidc_config to build the authentication state.
• Registers authentication-related routes under the /auth path.
• Attaches the authentication state as managed state in Rocket.

Returns the updated Rocket instance, or an error if the setup failed.