Automatic merge from submit-queue (batch tested with PRs 46033, 46122, 46053, 46018, 45981)
Command tree and exported env in kubectl plugins
This is part of `kubectl` plugins V1:
- Adds support to several env vars passing context information to the plugin. Plugins can make use of them to connect to the REST API, access global flags, get the path of the plugin caller (so that `kubectl` can be invoked) and so on. Exported env vars include
- `KUBECTL_PLUGINS_DESCRIPTOR_*`: the plugin descriptor fields
- `KUBECTL_PLUGINS_GLOBAL_FLAG_*`: one for each global flag, useful to access namespace, context, etc
- ~`KUBECTL_PLUGINS_REST_CLIENT_CONFIG_*`: one for most fields in `rest.Config` so that a REST client can be built.~
- `KUBECTL_PLUGINS_CALLER`: path to `kubectl`
- `KUBECTL_PLUGINS_CURRENT_NAMESPACE`: namespace in use
- Adds support for plugins as child of other plugins so that a tree of commands can be built (e.g. `kubectl myplugin list`, `kubectl myplugin add`, etc)
**Release note**:
```release-note
Added support to a hierarchy of kubectl plugins (a tree of plugins as children of other plugins).
Added exported env vars to kubectl plugins so that plugin developers have access to global flags, namespace, the plugin descriptor and the full path to the caller binary.
```
@kubernetes/sig-cli-pr-reviews
8fe818b2a1