feat: include template variables in dynamic parameter rendering (#18819)

Closes https://github.com/coder/coder/issues/18671

Template variables now loaded into dynamic parameters.

coderd/dynamicparameters/variablevalues.go

@@ -0,0 +1,65 @@
+package dynamicparameters
+
+import (
+	"strconv"
+
+	"github.com/zclconf/go-cty/cty"
+	"github.com/zclconf/go-cty/cty/json"
+	"golang.org/x/xerrors"
+
+	"github.com/coder/coder/v2/coderd/database"
+)
+
+// VariableValues is a helper function that converts a slice of TemplateVersionVariable
+// into a map of cty.Value for use in coder/preview.
+func VariableValues(vals []database.TemplateVersionVariable) (map[string]cty.Value, error) {
+	ctyVals := make(map[string]cty.Value, len(vals))
+	for _, v := range vals {
+		value := v.Value
+		if value == "" && v.DefaultValue != "" {
+			value = v.DefaultValue
+		}
+
+		if value == "" {
+			// Empty strings are unsupported I guess?
+			continue // omit non-set vals
+		}
+
+		var err error
+		switch v.Type {
+		// Defaulting the empty type to "string"
+		// TODO: This does not match the terraform behavior, however it is too late
+		// at this point in the code to determine this, as the database type stores all values
+		// as strings. The code needs to be fixed in the `Parse` step of the provisioner.
+		// That step should determine the type of the variable correctly and store it in the database.
+		case "string", "":
+			ctyVals[v.Name] = cty.StringVal(value)
+		case "number":
+			ctyVals[v.Name], err = cty.ParseNumberVal(value)
+			if err != nil {
+				return nil, xerrors.Errorf("parse variable %q: %w", v.Name, err)
+			}
+		case "bool":
+			parsed, err := strconv.ParseBool(value)
+			if err != nil {
+				return nil, xerrors.Errorf("parse variable %q: %w", v.Name, err)
+			}
+			ctyVals[v.Name] = cty.BoolVal(parsed)
+		default:
+			// If it is a complex type, let the cty json code give it a try.
+			// TODO: Ideally we parse `list` & `map` and build the type ourselves.
+			ty, err := json.ImpliedType([]byte(value))
+			if err != nil {
+				return nil, xerrors.Errorf("implied type for variable %q: %w", v.Name, err)
+			}
+
+			jv, err := json.Unmarshal([]byte(value), ty)
+			if err != nil {
+				return nil, xerrors.Errorf("unmarshal variable %q: %w", v.Name, err)
+			}
+			ctyVals[v.Name] = jv
+		}
+	}
+
+	return ctyVals, nil
+}