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.

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Install the latest version of @sveltejs/kit from 0eb057f:

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

[changeset-bot]

🦋 Changeset detected

Latest commit: 0eb057f

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

This PR includes changesets to release 4 packages

Name	Type
@sveltejs/kit	Minor
@sveltejs/adapter-vercel	Minor
@sveltejs/adapter-node	Minor
@sveltejs/adapter-cloudflare	Minor

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[svelte-docs-bot]

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