feat: add AuthUrl param for servercore compatibility

[ykachube]

Overview

This pull request adds support for configuring the authentication URL for the Selectel DNS provider. It allows users to specify a custom identity endpoint for the Selectel API service, which is especially beneficial for users of ServerCore API or private cloud deployments.
Problem

Currently, the webhook hardcodes the auth URL (https://cloud.api.selcloud.ru/identity/v3) when initializing the selvpcclient. This presents a problem for users who need to connect to different instances of the Selectel API, such as ServerCore or other private cloud environments.
Solution

This PR adds:

An authUrl field to the Config struct in selectel.go
Sets a default value in NewConfigForDNS() for backward compatibility
Passes this configuration to the selvpcclient.ClientOptions when creating the client
Also adds necessary AuthRegion parameter that's required by the client library