CallbackBuilder

[Keyrxng]

          Don't hesitate to move that to the SDK, it makes things much simpler!

This PR introduces a CallbackBuilder to replace the single handler input that SDK create methods use now. This is so a dev can create handlers for multiple webhook events and the SDK process them all as a single fn for any given event.

Proxy logic has been simplified via the new CallbackBuilder and the SDK should export it. (This task may need transferred into the SDK if it's no longer exported from this repo)

Originally posted by @gentlementlegen in ubiquity-os-marketplace/command-ask#9 (review)

[Keyrxng]

This should be transferred into https://github.com/ubiquity-os/plugin-sdk

Also if this is labelled I'm happy to re-sub the PR