Extension Proposal: ash_twirp

** What is the purpose of the extension? **
[twirp](https://github.com/keathley/twirp) is an RPC framework. Ash resources ought to be capable of generating the protobuf definitions that twirp runs off of.

** How would it extend the DSL? ***
I haven't looked into how we'd leverage it, but it was recommended by @keathley as a potentially good fit for Ash.

Start by reviewing the linked Twirp project and Ash's extension and DSL design. Clarify how Ash resources should generate the protobuf definitions required by Twirp and what DSL changes are needed; done would be an agreed implementation scope with defined outputs.

Written by the indexing model from the issue text.