Add indices and filter information to search shards api output
The search shards api returns info about which shards are going to be hit by executing a search with provided parameters: indices, routing, preference. Indices can also be aliases, which can also hold filters. The output includes an array of shards and a summary of all the nodes the shards are allocated on. This commit adds a new indices section to the search shards output that includes one entry per index, where each index can be associated with an optional filter in case the index was hit through a filtered alias.
This is relevant since we have moved parsing of alias filters to the coordinating node.
Relates to #20916
The `type` parameter has always been accepted by the search_shards api, probably to make the api and its urls the same as search. Truth is that the type never had any effect, it's been ignored from day one while accepting it may make users think that we actually do something with it.
This commit removes support for the type parameter from the REST layer and the Java API. Backwards compatibility is maintained on the transport layer though.
The new added serialization test also uncovered a bug in the java API where the `ClusterSearchShardsRequest` could be created with no arguments, but the indices were required to be not null otherwise the request couldn't be serialized as `writeTo` would throw NPE. Fixed by setting a default value (empty array) for indices.
Luca Cavanna
Nik Everett
Clinton Gormley