fix: Xml namespaces support

[istarkov]

Description

closes #4343

Steps for reproduction

https://p-cddc1d44-af37-4cb6-a430-d300cf6f932d-dot-sitemap.staging.webstudio.is/?pageId=FsnS9ui6btzM4W3YELE3Q&authToken=1cdc6026-dd5b-4624-b89b-9bd45e9bcc3d

Resulting xml

https://webstudio-fixture-project-a-0su3o.wstd.work/sitemap.xml

<?xml version="1.0" encoding="UTF-8"?>
<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9" xmlns:xhtml="http://www.w3.org/1999/xhtml"><url><loc>https://webstudio-fixture-project-a-0su3o.wstd.work/</loc><lastmod>2024-05-07</lastmod><xhtml:link rel="alternate" hreflang="en" href="https://webstudio-fixture-project-a-0su3o.wstd.work/en"></xhtml:link></url><url><loc>https://webstudio-fixture-project-a-0su3o.wstd.work/olegs-test</loc><lastmod>2024-05-07</lastmod><xhtml:link rel="alternate" hreflang="en" href="https://webstudio-fixture-project-a-0su3o.wstd.work/olegs-testen"></xhtml:link></url><url><loc>custom-hand-made-location</loc><lastmod>2020-10-10</lastmod><xhtml:link rel="alternate" hreflang="en" href="custom-en-location"></xhtml:link></url></urlset>

Code Review

• hi @kof, I need you to do
  • conceptual review (architecture, feature-correctness)
  • detailed review (read every line)
  • test it on preview

Before requesting a review

• made a self-review
• added inline comments where things may be not obvious (the "why", not "what")

Before merging

• tested locally and on preview environment (preview dev login: 5de6)
• updated test cases document
• added tests
• if any new env variables are added, added them to .env file