ILM: Additional filtering options for ILM explain API

[ppf2]

6.7.0

https://www.elastic.co/guide/en/elasticsearch/reference/master/ilm-explain-lifecycle.html

Currently, the ILM explain API provides an explanation for all indices regardless of whether they are managed by ILM or not.

It can be helpful to provide a few additional (optional) filtering parameters to this API (examples):

• managed=true|false so that admins can quickly filter out indices that are not managed by ILM to begin with
• error=true|false so that admins can quickly return just the indices that have step errors

[elasticmachine]

Pinging @elastic/es-core-features