Add option "--service-account-name" to install cmd (#5802)

The option "--service-account-name" is to be added to that user can use
an existing service account for velero and node-agent pods.  This is
helpful for users who wanna use IRSA.

Signed-off-by: Daniel Jiang <jiangd@vmware.com>

pkg/install/daemonset_test.go

@@ -40,4 +40,7 @@ func TestDaemonSet(t *testing.T) {
 	ds = DaemonSet("velero", WithFeatures([]string{"foo,bar,baz"}))
 	assert.Len(t, ds.Spec.Template.Spec.Containers[0].Args, 3)
 	assert.Equal(t, "--features=foo,bar,baz", ds.Spec.Template.Spec.Containers[0].Args[2])
+
+	ds = DaemonSet("velero", WithServiceAccountName("test-sa"))
+	assert.Equal(t, "test-sa", ds.Spec.Template.Spec.ServiceAccountName)
 }