Poc to add namespace to be kind of target - follow up on issue 165 (add gardenctl target namespace xxx)

[neo-liang-sap]

This issue is an followup issue from #165
In #165 , gardenctl target namespace xxx or gardenctl target -g xxx -s xxx -p xxx -t xxx -n xxx is supported.
The aim of this issue is to add namespace to be kind of target, like garden/shoot/seed/project.
The refactor will have impact on :

• gardenctl ls/get/drop/target operations
• getting current kubeconfig (garden/seed/shoot have kubeconfig, project value exist in kubeconfig path) but namespace doesn't have kubeconfig as namespace is not an cluster
• other issues i haven't met so far

More discussion could be found in #168 (comment) and #168 (comment)

[neo-liang-sap]

please assign this issue to me , thanks!