Support for Sitemap Index (to include other sitemaps) in sitemap.ts file

[sakib412]

Goals

1. Ability to add other sitemap links
2. Combine multiple splitted sitemap into one sitemap url
3. So user don't have to include multiple sitemap into search console.
4. Many SAAS app allow their client to manage dynamic sitemap configuration, so data can be fetched from db or other source to load either urlset or sitemapindex. that is not currently possible in sitemap.xml

Examples

<?xml version="1.0" encoding="UTF-8"?>
<sitemapindex xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
  <sitemap>
    <loc>https://example.com/sitemap-main.xml</loc>
  </sitemap>
  <sitemap>
    <loc>https://example.com/sitemap-posts.xml</loc>
  </sitemap>
</sitemapindex>

there is no system to generate sitemapindex output from sitemap.(js,ts)

Non-Goals

1. No need to combine urlset and sitemapindex into one file. it's not possible/supported afaik.
2. This is now possible only in app/sitemap.xml, so only need in dynamic sitemap generation (sitemap.ts)

Background

Many SAAS app allow their client to manage dynamic sitemap configuration, so data can be fetched from db or other source to load either urlset or sitemapindex. that is not currently possible in sitemap.xml

Proposal

Now sitemap only support urlset. so in top level or mode (can be named differently) set to either urlset or sitemapindex.
then either one will be allowed.