Labeler: Add support for assign

[mrbobbytables]

Eventually it would be good to be able to support /assign

[jaiakash]

Hi @mrbobbytables that would be good. I was testing this here https://github.com/jaiakash/automation/commit/9d2706184fe2bdf9cfa82a0b263e16d981b9739e

If this looks good, please assign me this issue. I still need to validate it using my own images instead of the CNCF image (ghcr.io/cncf/gha-labeler:assign-label → denied).

Nevertheless, I can test that and raise PR for same.