Publish crates to crates.io

[Mossaka]

The spin engine and triggers are highly reusuable crates. It will be great to publish them to crates.io so that we can use spin engine as a SDK.

For now there is a workaround:

spin-http-engine =  { git = "https://github.com/fermyon/spin", branch = "main" }
spin-loader =  { git = "https://github.com/fermyon/spin", branch = "main" }

[radu-matei]

ref #316

[joepio]

Personally I like installing things using cargo install, so even the CLI makes sense to publish to crates (there's already a placeholder btw)