Add _pipeline support to log event

[AlexKovalevich]

Add ability to use ES 5 ingest pipelines

_pipeline : "pipelineName" to the indexed data. Thais can be done either by making serializeEvents() or serializeIndexString() protected, or by providing additional configuration like:

			String pipeline = settings.getPipeline();
			if (pipeline != null) {
				gen.writeObjectField("_pipeline ", pipeline );
			}

[w39hh]

Although this is an old request, the following may still be of some use to someone.
We already make use of pipelines with the appender to convert strings to numbers, as we want some of the attributes in our log to be numeric.

We created a simple pipeline we called to_number. The processor converts certain fields we use in our logging (duration, count) from text to number.

{
  "processors": [
    {
      "convert": {
        "field": "duration",
        "type": "integer"
      }
    },
    {
      "convert": {
        "field": "count",
        "type": "integer"
      }
    }
  ]
}

After posting that pipeline to Elasticsearch, we configure the appender to use it as follows

   <appender name="ELASTIC" class="com.internetitem.logback.elasticsearch.ElasticsearchAppender">
      <url>${elastic.url}/_bulk/?pipeline=to_number</url>
      <index>nhcs-logback</index>
      <type>logback</type>
      <loggerName>es-logger</loggerName> <!-- optional -->
... etc