Merge pull request #42115 from justinsb/kubectl_expose_drainoptions

Automatic merge from submit-queue (batch tested with PRs 42162, 41973, 42015, 42115, 41923)

kubectl drain: make code reusable

DrainOptions requires a few fields to be set, and the expectation is
that these are set as part of construction of the object.  If they are
set, then the drain code can be reused in other kubernetes projects.

This does not create a contract that DrainOptions should fulfill going
forwards, any more than any of the other types that happen to be exposed
are part of the contract.  Instead, this merely makes use outside the
package possible.

```release-note
NONE
```

pkg/kubectl/cmd/drain_test.go

@@ -732,7 +732,7 @@ func TestDeletePods(t *testing.T) {
 		f, _, _, _ := cmdtesting.NewAPIFactory()
 		o := DrainOptions{}
 		o.mapper, _ = f.Object()
-		o.out = os.Stdout
+		o.Out = os.Stdout
 		_, pods := createPods(false)
 		pendingPods, err := o.waitForDelete(pods, test.interval, test.timeout, false, test.getPodFn)