Add labeled logic to auto cancel for PR builds

[ecrupper]

Description

Based on the current implementation of pull_request event types for auto cancel, labeled actions are not processed.

Some users may trigger builds via a label and would want to auto cancel those builds upon re-adding the label.

Value

Use case of triggering long-running label builds (test suites or the like) would be addressed by adding support for the label action.

Definition of Done

Vela handles pull_request:labeled as a potential auto-canceler. May need to add some options to the auto_cancel YAML map.

Effort (Optional)

1-2 days dev time

Impacted Personas (Optional)

responsible auto-cancelers