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

[Jacob-Roberts]

Note

This is moved from #12320 to point to the main branch

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.

I have added an test that ensures that the build output contains the base URL that we specify.

Docs

Docs are added in withastro/docs#10106 that explain to integration authors this new config option.

[changeset-bot]

🦋 Changeset detected

Latest commit: 1376964

The changes in this PR will be included in the next version bump.

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[github-actions]

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

[codspeed-hq]

CodSpeed Performance Report

Merging #12560 will degrade performances by 86.4%

_{Comparing Jacob-Roberts:Jacob-Roberts-dynamic-static-split-core (1376964) with main (c50882d)}

Summary

❌ 2 regressions
✅ 4 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	main	Jacob-Roberts:Jacob-Roberts-dynamic-static-split-core	Change
❌	Rendering: streaming [false], .astro file	1.5 s	1.8 s	-13.86%
❌	Rendering: streaming [false], .md file	22.4 ms	164.7 ms	-86.4%