feat(astro): override components to support `HeadTags`

[eric-burel]

I was investigating setting up a sitemap, using @astrojs/sitemap. For a proper setup, you also want to add a link tag in the page head to tell scrapers the URL to your sitemap (in addition to robot.txt). We could do this in the framework default setup, but that would mean setting up sitemap as an integration there and I'd rather give end users more control on the configuration.

So I went down the rabbit hole and implemented a replaceable HeadTags component.

I've tried to make the API as future proof as possible using slots.

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