Provide HTTP API for listing past deliveries

[joao-paulo-parity]

Problem: we have to bug GitHub org admins to get closure on past deliveries, as done in e.g. https://github.com/paritytech/internal_it/issues/67

Solution: Fairly recently GitHub made available endpoints for listing those deliveries: https://docs.github.com/en/rest/reference/apps#list-deliveries-for-an-app-webhook. Therefore we should expose an API for listing past deliveries so that this information can be queried without having to bother our GitHub org admins.

This is more important for bench-bot specifically, currently, we had multiple incidents related to the bot becoming unavailable due to certificate issues (https://github.com/paritytech/devops/issues/1492, https://github.com/paritytech/devops/issues/1612, https://github.com/paritytech/devops/issues/1070#issuecomment-1024446019) and so it's important to know if the events are reaching the server before we even get to debugging the application itself.