add a way to upgrade to a new version

[sirdarckcat]

kctf-upgrade or similar

[sirdarckcat]

with the operator, we now have three concepts of upgrades:

1. upgrade the operator
2. upgrade the challenge.yaml
3. upgrade the docker images

upgrading the challenge.yaml I think can be left out of scope for now (this can be a manual operation) so we just have upgrading the operator, and the docker images, which are properties of the SDK.

[sroettger]

There's also the gce project state. I.e. the changes we do to service accounts, etc can differ between kctf versions.
Most of the time it's probably fine to just apply the new changes (like, old unused service accounts are no issue) but I think it's possible that we change some state that we would have to revert during an upgrade.