feat: --strategy flag for context switching

[mircea-pavel-anton]

If strategy is set to merge, create a big kubeconfig with all the contexts defined in all kubeconfig files and set active context.

If strategy is set to copy, just copy over the one kubeconfig file with the wanted context and set active context.

This way, with the merge strategy (which can be the default?), things like passing --context to kubectl commands or selecting contexts in k9s would still work