Add label based on PR (title, body, commits)

To help with PR triage, we could add an action that parses the PR title, body and commits (opt-in) to check for information allowing us to add default / custom labels. For example, automatically adding the "breaking" label, specifying the priority of the PR, specifying if it is associated to an inner tracking tool, ...

This action should be complementary to `actions/labeler` and not checking the modified files. The checking of commits is to retrieve git trailers in order to add labels.

Extend the idea proposed in #757

Start by reading the proposal in #757 and comparing the intended behavior with actions/labeler. Determine the action's supported inputs and how it should inspect PR titles, bodies, and optionally commit trailers; done means it can apply default or custom labels without checking modified files.

Written by the indexing model from the issue text.