Runtime Interface / Custom Host Function

[JoshOrndorff]

Substrate has the notion of host functions. They are code that runs on the client-side and can be called by the runtime. The standard ones are for cryptography. It is possible to add your own host functions. The recipes should cover this.

@HammeWang has done an [example using sm2 cryptography](https://github.com/hammeWang/substrate-sm2_ and presented it on Substrate Seminar

@tgmichel has done a minimal example as learning for Moonbeam moonbeam-foundation/moonbeam@c5f5466

This is a nice couterpart to the Runtime API recipe. They are both examples of crossing the wasm boundary, but in opposite ways.