feat: Expose managed variables via API (#6134)

* WIP

* hcl

* useManagedVariables

* fix

* Fix

* Fix

* fix

* go:build

* Fix

* fix: bool flag

* Insert template variables

* API

* fix

* Expose via API

* More wiring

* CLI for testing purposes

* WIP

* Delete FIXME

* planVars

* WIP

* WIP

* UserVariableValues

* no dry run

* Dry run

* Done FIXME

* Fix

* Fix: CLI

* Fix: migration

* API tests

* Test info

* Tests

* More tests

* fix: lint

* Fix: authz

* Address PR comments

* Fix

* fix

* fix

cli/templatevariables.go

@@ -0,0 +1,50 @@
+package cli
+
+import (
+	"os"
+
+	"golang.org/x/xerrors"
+	"gopkg.in/yaml.v3"
+
+	"github.com/coder/coder/codersdk"
+)
+
+func loadVariableValues(variablesFile string) ([]codersdk.VariableValue, error) {
+	var values []codersdk.VariableValue
+	if variablesFile == "" {
+		return values, nil
+	}
+
+	variablesMap, err := createVariablesMapFromFile(variablesFile)
+	if err != nil {
+		return nil, err
+	}
+
+	for name, value := range variablesMap {
+		values = append(values, codersdk.VariableValue{
+			Name:  name,
+			Value: value,
+		})
+	}
+	return values, nil
+}
+
+// Reads a YAML file and populates a string -> string map.
+// Throws an error if the file name is empty.
+func createVariablesMapFromFile(variablesFile string) (map[string]string, error) {
+	if variablesFile == "" {
+		return nil, xerrors.Errorf("variable file name is not specified")
+	}
+
+	variablesMap := make(map[string]string)
+	variablesFileContents, err := os.ReadFile(variablesFile)
+	if err != nil {
+		return nil, err
+	}
+
+	err = yaml.Unmarshal(variablesFileContents, &variablesMap)
+	if err != nil {
+		return nil, err
+	}
+	return variablesMap, nil
+}