server: add OSS stubs supporting validation of source namespaces in service-intentions config entries

.changelog/9527.txt

@@ -0,0 +1,3 @@
+```release-note:improvement
+server: **(Enterprise Only)** Validate source namespaces in service-intentions config entries.
+```

agent/structs/config_entry_intentions.go

@@ -567,6 +567,10 @@ func (e *ServiceIntentionsConfigEntry) validate(legacyWrite bool) error {
 			return fmt.Errorf("Sources[%d].%v", i, err)
 		}
 
+		if err := validateSourceIntentionEnterpriseMeta(&src.EnterpriseMeta); err != nil {
+			return fmt.Errorf("Sources[%d].%v", i, err)
+		}
+
 		// Length of opaque values
 		if len(src.Description) > metaValueMaxLength {
 			return fmt.Errorf(

agent/structs/config_entry_intentions_oss.go

@@ -0,0 +1,7 @@
+// +build !consulent
+
+package structs
+
+func validateSourceIntentionEnterpriseMeta(_ *EnterpriseMeta) error {
+	return nil
+}