[new service] Automatically scrape OpenTTD releases and emit events

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This will probably be called service.versionwatch. It'll run on a fairly regular basis (probably every 10 minutes) and check the latest releases against a local data cache. If there's a newer version (or a change to an already known version, like if it gets promoted (even though this doesn't happen with OpenTTD)) it'll emit an event that can be caught by other services, like the build scheduler.