Vercel functions runtime integration

[oscartbeaumont]

Add an integration for Vercel Functions community maintained Rust runtime.

[oscartbeaumont]

For now, Netlify Functions support has been added and is recommended because my experience with the Vercel Rust Serverless and Edge Runtimes was not good.

[oscartbeaumont]

An example is in the repo and it works but it's rough around the edges.

We could probs also support Vercel's Edge runtime if I can be bothered.