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.go

@@ -0,0 +1,18 @@
+package fs
+
+import (
+	"os"
+)
+
+// Exists determines whether a file/directory exists or not.
+func Exists(fpath string) (bool, error) {
+	_, err := os.Stat(fpath)
+	if err != nil {
+		if !os.IsNotExist(err) {
+			return false, err
+		}
+		return false, nil
+	}
+
+	return true, nil
+}