How to define and implement a custom runtime API

[sacha-l]

Port over Recipe about Runtime APIs.

[nuke-web3]

Please also update devhub references when this is available: https://github.com/substrate-developer-hub/substrate-developer-hub.github.io/blob/source/docs/tutorials/frontier/frontier-architecture.md#ethereum-specific-runtime-apis--rpcs

[justinfrevert]

@HashWarlock was just asking me the other day about a code example for this. I think a code example, or a step-by-step implementation guide would pre-empt a lot of questions. With regards to the code example itself, it might be cool if there is a reasonable way to tie in custom host functions as well, so we can show communication both ways.