Allow override of destination Elasticsearch index on a per-doc basis

[p-kimberley]

Currently all Elasticsearch documents for a given ElasticIndexingFilter are sent to the same destination index (unless using a date-based pattern).

In certain use cases, it is desirable for the destination index to be overridden. An example of this is where events matching certain criteria are sent to a separate data stream for more rapid age-off (i.e. a different ILM policy is applied).

Consider testing for the existence of a root-level _index property, which if specified, overrides the doc index.