feat: allow additional handlers to be exported by adapter-cloudflare

[thomasfosterau]

closes #10117 closes #1712

Cloudflare Workers allows for handlers to be exported from a worker as methods on an object exported as a default export. Currently, adapter-cloudflare-workers exports a fetch handler method, but there is no way to add other handler methods, such as a scheduled or queue handler. This functionality was requested just over a year ago in #10496.

This PR adds a configuration option for @sveltejs/adapter-cloudflare to provide a file whose exports get added to the default option exported by the worker the adapter generates. The fetch handler does not get overridden.

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