Bug 2067851 - viaduct-hyper -> viaduct-backend-rust

[bendk]

The older hyper version we were using triggered RUSTSEC-2026-0258 when running cargo audit. I think switching back to reqwest makes more sense than upgrading to hyper 1.x Renamed the crate to viaduct-backend-rust since it's not using hyper anymore. See the bug comments for my reasoning around this.

I wasn't actually able to remove the RUSTSEC-2026-0258 exclusion, since nimbus-cli is still using hyper. However, I did remove the RUSTSEC-2018-0006 one. It's no longer needed since we're on clap v4.

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[bendk]

I don't love having to do this, but like I mentioned in the bug it would have also been a similar amount of work to upgrade to hyper 1.x. I'm thinking it's best to admit that using hyper was a mistake and go back to reqwest.

[bendk]

I ended up using reqwest 0.11 instead of 0.13 because I had issues with the dependency checks for 0.13. That made we wonder if this is the right solution going forward, but I still think it is because dependency checks are relatively simple to fix and we might not even have them once we move to the monorepo.