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/tmpl/expand_secrets_mock.go

@@ -0,0 +1,45 @@
+// Code generated by MockGen. DO NOT EDIT.
+// Source: expand_secret_ref.go
+
+package tmpl
+
+import (
+	gomock "github.com/golang/mock/gomock"
+	reflect "reflect"
+)
+
+// MockvalClient is a mock of valClient interface
+type MockvalClient struct {
+	ctrl     *gomock.Controller
+	recorder *MockvalClientMockRecorder
+}
+
+// MockvalClientMockRecorder is the mock recorder for MockvalClient
+type MockvalClientMockRecorder struct {
+	mock *MockvalClient
+}
+
+// NewMockvalClient creates a new mock instance
+func NewMockvalClient(ctrl *gomock.Controller) *MockvalClient {
+	mock := &MockvalClient{ctrl: ctrl}
+	mock.recorder = &MockvalClientMockRecorder{mock}
+	return mock
+}
+
+// EXPECT returns an object that allows the caller to indicate expected use
+func (_m *MockvalClient) EXPECT() *MockvalClientMockRecorder {
+	return _m.recorder
+}
+
+// Eval mocks base method
+func (_m *MockvalClient) Eval(template map[string]interface{}) (map[string]interface{}, error) {
+	ret := _m.ctrl.Call(_m, "Eval", template)
+	ret0, _ := ret[0].(map[string]interface{})
+	ret1, _ := ret[1].(error)
+	return ret0, ret1
+}
+
+// Eval indicates an expected call of Eval
+func (_mr *MockvalClientMockRecorder) Eval(arg0 interface{}) *gomock.Call {
+	return _mr.mock.ctrl.RecordCallWithMethodType(_mr.mock, "Eval", reflect.TypeOf((*MockvalClient)(nil).Eval), arg0)
+}