Plugins: Require signing of external back-end plugins (#24075)

* PluginManager: Require signing of external plugins

Co-authored-by: Marcus Efraimsson <marcus.efraimsson@gmail.com>
Co-authored-by: Diana Payton <52059945+oddlittlebird@users.noreply.github.com>

pkg/infra/fs/exists_test.go

@@ -0,0 +1,26 @@
+package fs
+
+import (
+	"github.com/stretchr/testify/require"
+	"io/ioutil"
+	"os"
+	"testing"
+)
+
+func TestExists_NonExistent(t *testing.T) {
+	exists, err := Exists("non-existent")
+	require.NoError(t, err)
+
+	require.False(t, exists)
+}
+
+func TestExists_Existent(t *testing.T) {
+	f, err := ioutil.TempFile("", "")
+	require.NoError(t, err)
+	defer os.Remove(f.Name())
+
+	exists, err := Exists(f.Name())
+	require.NoError(t, err)
+
+	require.True(t, exists)
+}