feat(assets): Allow customising the route of the image endpoint

[Princesseuh]

Changes

Changes the image endpoint config to be a bit more intuitive and allow customizing the route, a niche but sometimes useful usecase. Since this is on the image config, custom endpoints can respect the user's wish regarding where to put it without requiring a special config.

Fix #8847

Testing

Updated tests to use new config

Docs

Some of it is docs! It's autogenerated from config

[changeset-bot]

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Latest commit: e147e27

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[bluwy]

not using a magic undefined value to mean something

I have a slight preference to use undefined instead if you mean the endpoint.entrypoint property 🤔 You could argue that you're adding a magic "default" value too that technically can be a entrypoint path to a package or alias called default, but we changed that to mean our default value. undefined is more of "let Astro fallback to default handling".

[Princesseuh]

Using undefined is fine with me! I'll change it back.

[github-actions]

This PR is blocked because it contains a major changeset. A reviewer will merge this at the next release if approved.