Allow grok pattern to be passed in as a parameter

[Furb13]

I have a complex use case where I want to store meta data about an event being parsed by logstash in a dictionary. In there, some fields may need additional grok on one of their fields already parsed out by kv plugin. This is why I need to pass in the grok pattern from an existing filed (variable).

Discussion pertaining to this: https://discuss.elastic.co/t/how-to-get-grok-filter-from-variable/118511

[Furb13]

Closing issue to put under the grok plugin. Issue has been moved here.