feat(template): added secret template function (#1221)

* feat(tmpl): added fetchSecretValue template function

This adds a tmpl `fetchSecretValue` and `expandSecretRefs` function by:
- Adding:
    - `expandSecretRefs` function in tmpl package that uses vals
    package to fetch secrets
    - `fetchSecretValue` function in tmpl package like below but for
    single string value
    - gomock for tests purpose
- Changing:
    - move init of vals package to function (so the same instance can be used for template values and rendering the whole template)

* doc(secret): added doc how to use new tmpl methods

Added example usage of `fetchSecretValue` and `expandSecretRefs`

pkg/plugins/vals_test.go

@@ -0,0 +1,17 @@
+package plugins
+
+import "testing"
+
+func TestValsInstance(t *testing.T) {
+	i, err := ValsInstance()
+
+	if err != nil {
+		t.Errorf("unexpected error: %v", err)
+	}
+
+	i2, _ := ValsInstance()
+
+	if i != i2 {
+		t.Error("Instances should be equal")
+	}
+}