Admission controller for configuration validation

[makhov]

We are thinking about having an admission controller or CEL validation. In this issue, we are collecting a list of things that we should be validating:

• don't allow to create k0s clusters in --single mode and with updateStrategy=Recreate
• ensure kubernetes skew policy is respected
• …

[apedriza]

• CRDs validations on references to other resources within the CRD (ObjectRef exists etc...). In this way we can lighten the controller processing and help in the debugging for the user. For example: do not allow to create a JoinTokenRequest referencing a non-existent Cluster.