feat: Allow integrations to change the base for server-islands URLs

[Jacob-Roberts]

See proposal withastro/roadmap#1043 to see the full reasoning behind this change.

This PR only contains the required changes to enable an integration to be built. However, I created a draft PR that shows an integration and the example that uses it #12304. Please look at that PR to see the full desired impact of this change.

Changes

• Allow changing the domain that the server islands are hosted on.
• Allow developers to split the "static" part and the "dynamic" part of their website fully.
• Allows developers that cannot give Cloudflare the ability to read their customer's data to still use Astro's great site builder.
• Use a specific integration so we don't pollute the main astro config.

With config:

Result:

Testing

A manual test has been run in the separate PR. No automated tests have been added.

Because this is a niche use-case I wanted to get feedback from the team before adding tests for it. If it is desirable, I will add an e2e test that creates a new server island fixture to test this.

Docs

No docs are needed for this as this is a very niche configuration option. We don't want to clutter the main astro config, hence why I didn't add the config to the "public" user-facing config. If this is something we want to override, let's only allow it to be overriden from an integration level.

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[ematipico]

@Jacob-Roberts

If you're looking for feedback, please make the PR a draft. As you said, the PR isn't meant to be ready because tests are missing.

Also, regardless of the entity of the feature, docs must be provided to the user.