feat: allow adapters to override getRequest and setResponse

[ottomated]

This enables the cloudflare adapter to emulate the Request.cf property and later handle websockets responses.

---

Please don't delete this checklist! Before submitting the PR, please make sure you do the following:

• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.

Tests

• Run the tests with pnpm test and lint the project with pnpm lint and pnpm check

Changesets

• If your PR makes a change that should be noted in one or more packages' changelogs, generate a changeset by running pnpm changeset and following the prompts. Changesets that add features should be minor and those that fix bugs should be patch. Please prefix changeset messages with feat:, fix:, or chore:.

[pkg-svelte-dev]

Install the latest version of @sveltejs/kit from f24c7fc:

pnpm add https://pkg.svelte.dev/@sveltejs/kit/c/f24c7fced27e08176e5b6499d2a4909ea90e3f33

Open in pkg.svelte.dev: https://pkg.svelte.dev/repos/kit/pr/16753

[changeset-bot]

🦋 Changeset detected

Latest commit: f24c7fc

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

This PR includes changesets to release 1 package

Name	Type
@sveltejs/kit	Minor

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

[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-kit-16753-svelte.vercel.app/