feat(server-island): Add ability to specify the domain that server islands are hosted on

[Jacob-Roberts]

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

My ultimate goal is to make this an integration, but as it stands, I haven't been able to make it work fully as a plugin without changing the core. So I'm very open to feedback.

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

Currently just a manual test has been run. No automated tests have been added.

TODO: Add automated tests.

Docs

TODO: Doc updates

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Latest commit: 72d432f

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[Jacob-Roberts]

Closing because this has examples/integrations that won't make it into the real repository. They will go in a separate repo and be published on their own.