Auto-copy labels from original PR

[ssbarnea]

It would really be useful to copy all labels from source PRs with the exception of the triggering ones (backport).

[webknjaz]

I think that we need to be careful about how this is implemented. This may need allow-/block-list in the config because blindly copying the labels may trigger other unwanted integrations. The bot may even loop on its own if proper filtering is not applied.