feat: support custom URLs to be added to the Sitemap

[johnedvard]

Summary

Add support to define custom URLs to be used when generating the Sitemap.

I came across this useful plugin, but it doesn't work for Single Page Applications (SPA.) I thought it would be great to support adding custom URLs to the Sitemap via the plugin.

I think these changes will partly solve https://github.com/netlify-labs/netlify-plugin-sitemap/issues/8. Though these changes don't crawl the page, nor try to guess the routes used for the SPA, I think the solution is useful for developers looking for a simple way of adding a few URLs to their sitemap.

I'll put the PR in draft to begin with, because I want to get your opinions of my suggestion. I'm also not sure about what lastmodfile is used for, so I just omitted it when generating the customURLs.