Create an informer in instance manager to cache custom resources changes

[yuxuanzhu0214]

Is this a BUG REPORT or FEATURE REQUEST?: FEATURE REQUEST

What happened: We want to have a controller that watches for changes in instance manager CR. One way is to rely on API server and poll periodically for all instance manager CR, but this increases the load on API server. The other way is a informer that caches the changes of instance manager CR and this event driven method reduces load on API server.

What you expected to happen: The informer can be generated using the sample script update-codegen.sh in this repo: https://github.com/kubernetes/sample-controller/blob/master/hack/update-codegen.sh