fix: Automatic I18n Multi Sitemap + custom sitemaps not working

[okainov]

🐛 The bug

Documentation currently mentions that

Automatic I18n Multi Sitemap
The module automatically generates a sitemap for each locale when:

You're not using the no_prefix strategy
Or you're using Different Domains
And you haven't manually configured the sitemaps option

And that indeed works this way. But I'd like it to actually work with "manually configured the sitemaps option", maybe with some additional options

Example:

    sitemaps: {
      default: {
        // This should be absolutely the same as default multi-lang sitemap (actually, two sitemaps for each language)
        includeAppSources: true,
        exclude: ['/admin/**'],
      },
      places: {
        // And this is my custom dynamic sitemap 
        sources: [
          '/api/__sitemap__/urls',
        ]
      }
    }

Alternatively, what I miss is possibility to use both of _sitemap and _i18nTransform: true options for SitemapUrl.

      const entry: SitemapUrl = {
        _sitemap: "posts", // Kind of "sitemap prefix"
        loc: `${url}`,
        _i18nTransform: true,
      }

I would like this to produce one entry in "posts-en" sitemap and one in "posts-ru" (by amount of languages I have configured).

🛠️ To reproduce

N/A

🌈 Expected behavior

It would be great to get benefits of both worlds - automatic i18n-sitemap generation for the "main" website pages, as well as custom multi-lang sitemap for pages provided from external API (when I have thousands of "posts" or so)

ℹ️ Additional context

No response