Ability to override hreflang IDs

Problem/Motivation

In my drupal website, the country prefix is not same with sitemaps hreflang country code. So I need to create custom hreflang code

Steps to reproduce

Check in web\modules\contrib\simple_sitemap\src\Plugin\simple_sitemap\SitemapGenerator\DefaultSitemapGenerator

Proposed resolution

Create custom lang code

Remaining tasks

User interface changes

API changes

Data model changes