Support time.Time annotation values

The new yaml library for values formated as date/timestamps now returns
a `time.Time` typed value, this change makes sure we support it.

internal/opa/rule/rule.go

@@ -20,6 +20,7 @@ import (
 	"fmt"
 	"regexp"
 	"strings"
+	"time"
 
 	"github.com/open-policy-agent/opa/ast"
 )
@@ -48,9 +49,16 @@ func customAnnotationString(a *ast.AnnotationsRef, fieldName string) string {
 	if a == nil || a.Annotations == nil || a.Annotations.Custom == nil {
 		return ""
 	}
-	if value, ok := a.Annotations.Custom[fieldName].(string); ok {
-		return value
+
+	if value, ok := a.Annotations.Custom[fieldName]; ok {
+		switch value := value.(type) {
+		case string:
+			return value
+		case time.Time:
+			return value.Format(time.RFC3339)
+		}
 	}
+
 	return ""
 }
 

internal/opa/rule/rule_test.go

@@ -285,6 +285,16 @@ func TestEffectiveOn(t *testing.T) {
 		},
 		{
 			name: "with effective_on annotation",
+			annotation: annotationRef(heredoc.Doc(`
+				package a
+				# METADATA
+				# custom:
+				#   effective_on: 2022-01-01T00:00:00Z
+				deny() { true }`)),
+			expected: "2022-01-01T00:00:00Z",
+		},
+		{
+			name: "with effective_on annotation as string",
 			annotation: annotationRef(heredoc.Doc(`
 				package a
 				# METADATA