Command to download all missing inputs

[AV3RG]

As many users here rely on GitHub to store their past solutions, adding a command like cargo input-sync to download all missing inputs for the current year would be a valuable feature. Since inputs shouldn't be stored in the repository, this would help users setting up the project on new systems or running benchmarks on remote hardware without needing separate scripts for downloading missing inputs.

I've already implemented this for personal use and would be happy to create a PR if the maintainers think this would be a useful addition

[fspoettel]

Good idea, would accept a PR for this.