cirrus: Add smoke testing for builtin plugins

This adds two example plugins within testing/builtin-plugins/Files:

* protocol-plugin copied over from testing/btest/plugins/protocol-plugin

* py-lib-plugin that embeds Python to have a dependency on an external
  shared library which was already available in CI and fun to use, too.

Closes #2837

testing/builtin-plugins/Files/py-lib-plugin/plugin/src/Plugin.cc

@@ -0,0 +1,34 @@
+#include "Plugin.h"
+
+extern "C"
+	{
+#include <Python.h>
+	}
+
+namespace zeek::plugin::Zeek_PyLib
+	{
+Plugin plugin;
+	}
+
+using namespace zeek::plugin::Zeek_PyLib;
+
+zeek::plugin::Configuration Plugin::Configure()
+	{
+	zeek::plugin::Configuration config;
+	config.name = "Zeek::PyLib";
+	config.description = "Plugin embedding Python, whoosh.";
+	config.version.major = 0;
+	config.version.minor = 0;
+	config.version.patch = 1;
+	return config;
+	}
+
+void Plugin::InitPostScript()
+	{
+	Py_Initialize();
+	}
+
+void Plugin::Done()
+	{
+	Py_FinalizeEx();
+	}

testing/builtin-plugins/Files/py-lib-plugin/plugin/src/Plugin.h

@@ -0,0 +1,21 @@
+#pragma once
+
+#include <zeek/plugin/Plugin.h>
+
+namespace zeek::plugin
+	{
+namespace Zeek_PyLib
+	{
+
+class Plugin : public zeek::plugin::Plugin
+	{
+protected:
+	zeek::plugin::Configuration Configure() override;
+	void InitPostScript() override;
+	void Done() override;
+	};
+
+extern Plugin plugin;
+
+	}
+	}

testing/builtin-plugins/Files/py-lib-plugin/plugin/src/pylib.bif

@@ -0,0 +1,18 @@
+module Python;
+
+%%{
+extern "C" {
+#include <Python.h>
+}
+%%}
+
+function version%(%): string
+%{
+	return zeek::make_intrusive<zeek::StringVal>(Py_GetVersion());
+%}
+
+function run_simple_string%(s: string%): bool
+%{
+	PyRun_SimpleString(s->CheckString());
+	return zeek::val_mgr->True();
+%}