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.

[mordamax]

@pavelsupr i recently had another case where I was lucky to have a prod access to a webhooks in Github Application (try-runtime-bot) to be able to react quickly on prod issues, so im pretty sure our team would benefit of having access there, or loss of not having it

so im curious - is there a way to grant some minimum (may be there's granular ACL for this) level of access for prod GH apps that our team can get access too?
like we won't be able to kill the app, or change the settings without IT team approval, but at least read and attempt to change it
the secrets anyway hidden, so ¯\(ツ)/¯ what would go wrong if we do have it?