Debug an ingest node pipeline using an indexed document

[cjcenizal]

Per discussion with @alisonelizabeth and @jethr0null, we can provide a UI in the ingest node pipeline wizard that lets the user specify an index name and document ID, for selecting a document to test against the pipeline they're building. Some ideas for requirements:

• Cross-link to Discover. Kibana already provides tools for finding documents, so we'll want to point the user to those tools if they don't already know the document they want to use. We can provide some help text explaining what Discover is used for and offer a cross-link to it.
• Easy copy-pasting. If users are coming back to this UI from Discover or from Console, what's the easiest way for them to transfer the index name and document ID into this UI? Would they want to paste something in the form of <index>/_type/<doc_id>? Something else?
• Document preview. Users will want to verify that the document they've selected is indeed the one they want. Should we show them JSON? Tabular data, like in Discover? Cross-link back to the document in Discover?

[elasticmachine]

Pinging @elastic/es-ui (Team:Elasticsearch UI)