/_plugins resource schema enhancement

From https://github.com/elastic/logstash/pull/4794#issuecomment-197236928

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For debugging purpose this is quite useful. However now that I think about it, what we can extract from the gemspec is the requirement list, not the actual version of the gems installed..
This would be awesome: :dependencies => { "manticore" => { :depends => "~> 0.5.3", :installed => "0.5.6" }}

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@jsvd This is a good idea, lets make an issue to track it. I am not sure of the complexity to implement that feature and I think it should be hidden behind a query params.
I don't see it to be useful other than debugging the dependencies issues when something is wrong with the plugin manager.

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The feature is good, I'm all in for having an issue and implementing it in
a feature iteration.

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```

Proposal by @jsvd