[ML] Allow `indices_options` parameter for datafeed configuration

[benwtrent]

In certain situations it is valuable to allow missing indices (e.g. a well defined datafeed configuration that takes advantage of ECS). Right now, if one of the indices in the supplied index pattern does not exist, we throw an error on _start. If we exposed an indices_options type of parameter (that serializes to an IndicesOptions object), that will allow more advanced users to provide custom expansion options to handle their use case.

[elasticmachine]

Pinging @elastic/ml-core (:ml)

[jasonrhodes]

Related issue we're having on the logs/ML integration side: elastic/kibana#48219

[droberts195]

One particular effect of not being able to configure indices options on the datafeed config is that it's currently impossible to configure a datafeed to search frozen indices because you have to explicitly set the ignore_throttled=false option to do this.