Add more helper func for topologymutation func

[lubronzhan]

What would you like to be added (User Story)?

More helper func in topologymutation variable

Detailed Description

Now the consumer needs to add helper function to unmarshal object from cluster topologymutation variable. It would be helpful to have more helper func in upstream to do it.

Anything else you would like to add?

No response

Label(s) to be applied

/kind feature
One or more /area label. See https://github.com/kubernetes-sigs/cluster-api/labels?q=area for the list of labels.

[killianmuldoon]

/triage accepted

Thanks for tackling this!