kubeadm allows you to pass all the kinds in the same yaml muilti doc. prints out a warning for init config if you pass it on join. that's how k-sigs/kind passes them.
is there a case where someone might define both the init and join configuration in the capi construct?

i'm trying to understand if this is the best method if a config is "for join".

In our use case (KCP) the KubeadmConfigs we create for join always have joinConfiguration set and the ones for init don't have it. So this is the best way I'm aware of to figure out if a KCP KubeadmConfig is for a join.

is there a case where someone might define both the init and join configuration in the capi construct?

KCP would never do it, but other folks might do whatever they want. So probably this should not be a method on the API type and instead just a local func in the places where we use it

Moved to local private funcs