Add needs-rebase label to PRs that need to be rebased on main/master

[dippynark]

Even when a repository is configured to require a rebase before merging, this is not respected by keeper and so it repeatedly errors trying to merge.

Adding support for setting the needs-rebase label for PRs that need to be rebased would fix this since you can add needs-rebase to the tide/keeper missingLabels config.

Grepping through the code, the label is there but never actually used:

$ grep -r NeedsRebase .
./pkg/labels/labels.go:	NeedsRebase     = "needs-rebase"

[msvticket]

lighthouse is a fork of prow and the prow delegates the needs-rebase functionality to an external plugin
https://github.com/kubernetes/test-infra/tree/master/prow/external-plugins/needs-rebase

I think it would be quite easy to convert that to an external plugin for lighthouse.