Provide a general purpose API for extended plugin configuration

Currently plugins like the `grok` filter use files for extended configuration options like https://www.elastic.co/guide/en/logstash/current/plugins-filters-grok.html#plugins-filters-grok-patterns_dir to handle some options. These things are hard to configure centrally however. Ideally there'd be a way to centralize these options, perhaps by letting them be satisfied by an arbitrary URL, not just a central file.