Support RFC7231 UserAgent scheme in User Agent processor.

[Mpdreamz]

https://tools.ietf.org/html/rfc7231#section-5.5.3 defines user agent headers as:

     User-Agent = product *( RWS ( product / comment ) )

Example:
     User-Agent: CERN-LineMode/2.15 libwww/2.17b3

However the UA processor does not seem to support this OOTB

PUT _ingest/pipeline/user_agent
{
  "description" : "Add user agent information",
  "processors" : [
    {
      "user_agent" : {
        "field" : "agent"
      }
    }
  ]
}


PUT my_index/_doc/1?pipeline=user_agent&refresh=true
{
  "agent": "CERN-LineMode/2.15 libwww/2.17b3"
}

GET my_index/_doc/1

yields:

# GET my_index/_doc/1
{
  "_index" : "my_index",
  "_type" : "_doc",
  "_id" : "1",
  "_version" : 1,
  "_seq_no" : 0,
  "_primary_term" : 1,
  "found" : true,
  "_source" : {
    "agent" : "CERN-LineMode/2.15 libwww/2.17b3",
    "user_agent" : {
      "original" : "CERN-LineMode/2.15 libwww/2.17b3",
      "name" : "Other",
      "device" : {
        "name" : "Other"
      }
    }
  }
}

It would be nice if version would be set to 2.15 and name to CERN-LineMode without requiring a custom ua parser file.

[elasticmachine]

Pinging @elastic/es-core-features

[dakrone]

This has been open for quite a while, and we haven't made much progress on this due to focus in other areas. For now I'm going to close this as something we aren't planning on implementing. We can re-open it later if needed.