[Example] Support Coveo metadata

[kjvarga]

An example of how to support Coveo metadata https://docs.coveo.com/en/2656/index-content/index-xml-sitemap-metadata

Here's an example sitemap

SitemapGenerator::Sitemap.default_host = "http://www.example.com"
SitemapGenerator::Sitemap.compress = false
SitemapGenerator::Sitemap.create do
  add '/contents', coveo: { case_number: '12345', company_name: 'Publication Name' }
end

And output

<?xml version="1.0" encoding="UTF-8"?>
<urlset xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.sitemaps.org/schemas/sitemap/0.9 http://www.sitemaps.org/schemas/sitemap/0.9/sitemap.xsd"
  xmlns="http://www.sitemaps.org/schemas/sitemap/0.9"
  xmlns:image="http://www.google.com/schemas/sitemap-image/1.1"
  xmlns:video="http://www.google.com/schemas/sitemap-video/1.1"
  xmlns:news="http://www.google.com/schemas/sitemap-news/0.9"
  xmlns:mobile="http://www.google.com/schemas/sitemap-mobile/1.0"
  xmlns:pagemap="http://www.google.com/schemas/sitemap-pagemap/1.0"
  xmlns:xhtml="http://www.w3.org/1999/xhtml"
  xmlns:coveo="https://www.coveo.com/en/company/about-us">
  <url>
    <loc>http://www.example.com</loc>
    <lastmod>2022-08-15T11:31:53-07:00</lastmod>
    <changefreq>weekly</changefreq>
    <priority>1.0</priority>
  </url>
  <url>
    <loc>http://www.example.com/contents</loc>
    <lastmod>2022-08-15T11:31:53-07:00</lastmod>
    <changefreq>weekly</changefreq>
    <priority>0.5</priority>
    <coveo:metadata>
      <casenumber>12345</casenumber>
      <company_name>Publication Name</company_name>
    </coveo:metadata>
  </url>
</urlset>