[FEATURE] Copy applicable labels from original PR when backporting.

[Swiddis]

Is your feature request related to a problem?
When the backport action backports a PR that may have several labels, that information is lost in the backport unless manually re-added. For changelog work within our team, it's important for us to have appropriate changelog labels for all merged PRs, but it can be easy to forget to copy the labels from an original PR (example) to its backport (example).

What solution would you like?
An optional flag in the action that will copy any labels present on the PR, skipping any backport labels. Something like copy_labels_to_backport: boolean?

What alternatives have you considered?
N/A

Do you have any additional context?
N/A

[Swiddis]

Looks like there's a similar issue on the upstream repo that's also open. They hint at a solution with existing functionality, might experiment with it later.

[VachaShah]

This would be a great addition @Swiddis! Please feel free to raise a PR if you end up working out a solution. Thank you!