POD coverage tests for plugins

[racke]

If you run all_pod_coverage_ok(); on a Dancer2 plugin, you will get an output like the following:

# Coverage for Dancer2::Plugin::OpenAPI is 0.0%, with 15 naked subroutines:
# 	BUILD
# 	ClassHooks
# 	PluginKeyword
# 	dancer_app
# 	execute_plugin_hook
# 	hook
# 	keywords
# 	on_plugin_import
# 	plugin_args
# 	plugin_setting
# 	register
# 	register_hook
# 	register_plugin
# 	request
# 	var

I suggest to add a solution to the Dancer2::Plugin documentation, e.g.:

pod_coverage_ok(
    "Dancer2::Plugin::OpenAPI",
    {
        also_private => [
            qw/
              BUILDARGS BUILD ClassHooks PluginKeyword dancer_app
              execute_plugin_hook hook keywords on_plugin_import plugin_args
              plugin_setting realms realm realm_providers register register_hook
              register_plugin request var
              /
        ]
    });