btest/plugin: Add plugin testing enum identifiers

This plugin has a generic name, but for now just tests the API around
enum types and enum vals at InitPostScript() time.

testing/btest/plugins/api-plugin/src/Plugin.h

@@ -0,0 +1,18 @@
+
+#pragma once
+
+#include <zeek/plugin/Plugin.h>
+
+namespace btest::plugin::Demo_API {
+
+class Plugin : public zeek::plugin::Plugin {
+protected:
+    // Overridden from zeek::plugin::Plugin.
+    zeek::plugin::Configuration Configure() override;
+
+    void InitPostScript() override;
+};
+
+extern Plugin plugin;
+
+} // namespace btest::plugin::Demo_API