Add rebase github action

[Callisto13]

I really dislike the Update branch button on PRs. It leaves us with a really messy commit history.

Apparently this is a common complaint: isaacs/github#88 and isaacs/github#1113 (comment)

Also the Update branch is not accessible to the person who owns the PR.. not that I really want anyone clicking it.

What do we think about adding https://github.com/cirrus-actions/rebase? We could configure it so that the core team + the PR owner can add a /rebase comment to a PR and boom: lovely commit history.

[Callisto13]

wdyt @weaveworks/eksctl-reviewers ?

[aclevername]

I don't have any strong preferences either way 😄

[michaelbeaumont]

I'm fine with it but I think we should be squash + merging PRs anyway so I don't think this should affect the history very much either way

[Callisto13]

🤔 alright let's talk about enforcing that as a policy. I am concerned that it will lead to large commits which make debugging/bisecting harder in the future.