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Update the kubectl plugin mechanism
**Release note**:
```release-note
The plugin mechanism functionality to closely follow the git plugin design
```
Replace the existing plugin mechanism with the design proposed in https://github.com/kubernetes/community/pull/2437.
~~_The full implementation of the plugin mechanism itself is entirely contained within the first commit._~~
## Walkthrough
Under the new design, there is no plugin installation or loading required to use plugins.
A plugin is simply any executable file on a user's PATH whose name begins with `kubectl-`.
- Plugins receive the inherited environment from the `kubectl` binary. All environment variables
accessible by `kubectl` become accessible by the plugin.
- Plugins decide which command path they wish to implement based on their name. For example, a plugin wanting to provide a new command `foo`, would simply be named `kubectl-foo`.
### Creating a plugin
Below is an example plugin, that we will use for this walkthrough. Plugins may be written in any language, and handle arguments and flags in any way, optionally (as a convention) providing a way to retrieve their version via a `version` subcommand.
```bash
#!/bin/bash
# optional argument handling
if [[ "$1" == "version" ]]
then
echo "1.0.0"
exit 0
fi
# optional argument handling
if [[ "$1" == "config" ]]
then
echo $KUBECONFIG
exit 0
fi
echo "I am a plugin named kubectl-foo"
```
### Using a plugin
To use a plugin, simply make it executable:
```bash
sudo chmod +x ./kubectl-foo
```
and place it anywhere in your PATH:
```bash
sudo mv ./kubectl-foo /usr/local/bin
```
You may now invoke your plugin as a `kubectl` command:
```bash
$ kubectl foo
I am a plugin named kubectl-foo
```
All args and flags are passed as-is to the executable:
```bash
$ kubectl foo version
1.0.0
```
All environment variables are also passed as-is to the executable:
```bash
$ export KUBECONFIG=~/.kube/config
$ kubectl foo config
/home/<user>/.kube/config
$ KUBECONFIG=/etc/kube/config kubectl foo config
/etc/kube/config
```
Additionally, the first argument that is passed to a plugin will always be the full path to the location where it was invoked (`$0` would equal `/usr/local/bin/kubectl-foo` in our example above).
### Plugin discoverability
Seeing as how the `kubectl plugin` command is left as a no-op with this PR (perhaps it could serve as an entrypoint towards additional plugin functionality in the future), a small subcommand has been included that _lists all available plugin executables on a user's PATH_, along with any warnings it finds.
Example usage of this new subcommand is included below:
```bash
$ kubectl plugin list
The following kubectl-compatible plugins are available:
test/fixtures/pkg/kubectl/plugins/kubectl-foo
plugins/kubectl-foo
- warning: plugins/kubectl-foo is overshadowed by a similarly named plugin: test/fixtures/pkg/kubectl/plugins/kubectl-foo
plugins/kubectl-invalid
- warning: plugins/kubectl-invalid identified as a kubectl plugin, but it is not executable
plugins/kubectl-bar
error: 2 plugin warnings were found
```
cc @kubernetes/kubectl-maintainers @kubernetes/sig-cli-pr-reviews @soltysh @seans3 @mengqiy
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