workflow and actions for automatically labelling PRs based on changelog fragments

[tremble]

Originally written for amazon.aws: ansible-collections/amazon.aws#2454

As has been recently noticed, we're being inconsistent with backporting things.

Based on the keys from the changelog fragments this action would perform the following actions:

• On "push" or applying the mergeit label
  • If a PR contains non-backportable changes (major_changes, breaking_changes, removed_features) the do_not_backport label will be applied. These change types are assumed to require a "major" release, and as such shouldn't be backported into a currently "stable" release. At the same time it will remove any previously applied backport-* labels.
• When the "mergeit" label is applied:
  • If a PR contains changes that only need a minor release (minor_changes, deprecated_features) the backport-9 label will be applied.
  • If a PR contains changes that only need a patch release, such as security/bug fixes (bugfixes, security_fixes), then backport-8 and backport-9 labels will both be applied.

the labels to apply are configurable, and a (temporary) example of the workflow can be found at https://github.com/tremble/amazon.aws/blob/39c297ed743c1b0eb81dd93c3f531b3ba9492918/.github/workflows/backports.yml and tremble/amazon.aws#11

[tremble]

@abikouo split this off as requested.