[PoC] In-code kubectl apply

[mikkeloscar]

Proof of Concept of porting the "kubectl apply" code from deployment-service to also utilize it in CLM. Basically this means making the kubectl apply operation via Go instead of calling the external kubectl binary.

The motivation for this change is that we currently restrict how many executions of kubectl apply we do concurrently as it's hard to control the memory. This has the side effect of limiting how many clusters we can process in parallel. The idea is that by having this in go we save memory by not having to run another process and we thus can do more in parallel.

TODO

• Roll out behind a config-item to test in selected clusters first
• Check if we still need to control concurrency.