Function validate_authorization_server 

pub fn validate_authorization_server(
    metadata: &Value,
    issuer: &str,
    pds_url: &str,
    auth_method: &str,
) -> Result<AuthorizationServer>

Validate /.well-known/oauth-authorization-server and return its endpoints.

issuer is the URL the document was fetched from; pds_url the PDS that named it; auth_method the client-authentication method we intend to use (none for the localhost dev client, private_key_jwt in production).