Consider adding support for kubectl discovery cache

[tintoy]

Caches live at $HOME/.kube/cache/discovery/${KUBE_API_HOST}_${KUBE_API_PORT}.

• Inside this directory there is a servergroups.json file that contains a v1/APIGroupList with the cached API groups and their versions.
• Also inside this directory are subdirectories of the form $API_GROUP_NAME/$API_VERSION, each of which have a serverresources.json file containing a v1/APIResourceList with the cached API resources (i.e. resource types and their corresponding API paths).

Need to investigate the stability of the cache format (as long as the schema matches the APIGroupListV1 and APIResourceListV1 models, we're probably good to go).

CC: @felixfbecker this might help with the dynamic client (since we can preload the cache and make it read-through for anything not already cached).