feat(ci): open bump issues on consumer repos when a package is released

[valentinfernandez1]

What

A release opens a bump issue on every repository that consumes one of the packages it published. .github/consumers.json maps a package to its consumer repos, so subscribing a repo to another package is one entry in a list. Example:

{
  "@parity/truapi": ["paritytech/dotli-community", "paritytech/product-sdk"],
  "@parity/truapi-host": ["paritytech/dotli-community"],
  "@parity/ios-host": [],
  "@parity/android-host": []
}

A release publishing several packages a repo consumes produces one combined issue, labelled truapi-release. An earlier notification is closed as superseded only once the new issue covers every package it named at an equal or higher version, so a @parity/truapi-only release leaves an issue still tracking an unbumped @parity/truapi-host open. A release cut from a release branch cannot close the issue for a newer version.

Needs a GitHub App installed on the consumer repos, with its client id and private key stored as CONSUMER_NOTIFY_APP_CLIENT_ID and CONSUMER_NOTIFY_APP_PRIVATE_KEY. Each run mints an installation token scoped to the repos resolved from the config and to issues: write alone.

[valentinfernandez1]

CONSUMER_NOTIFY_TOKEN is set, the token is still pending approval so I will not merge until the token is approved.