Add support for the `cf.image` field in `fetch()` properties

[jakubadamw]

Per the definition in https://github.com/cloudflare/workerd/blob/4fb4dbd88d30ff293267680d85a31ddef94fd9b6/types/defines/cf.d.ts#L171-L295.

This is based on the existing https://github.com/cloudflare/workers-rs/pull/233 authored by @seeekr, with some improvements and the completion of the set of properties supported.

[jakubadamw]

@zebp any chance this could be reviewed? 🙂

Seems like there generally is a lot of outstanding PRs open and ready to be reviewed. I wonder what the best process of getting one's PR reviewed and merged is here? I think it would make sense to perhaps document the process in the README or, otherwise, external contributors may get discouraged from getting involved, seeing their contributions not picked up for review long after submission? (General observation, nothing personal!)