[series/9.x] add shard preference support for update by query async requests

[flavienbert]

Add support for the preference=_shards:0,1,2,3 parameter in async update-by-query operations through a new ShardsPreferenceRequest case class.

Changes:

• Add ShardsPreferenceRequest case class to model shard preferences. The goal of this model is to have a nonEmptyList
• Add shards: Option[ShardsPreferenceRequest] field to UpdateByQueryAsyncRequest
• Add fluent API methods: shards(Int) and shards(Set[Int])
• Propagate shard preference parameter to HTTP request in UpdateByQueryHandler
• Add comprehensive unit tests validating preference parameter building

[Philippus]

As far as I can tell, shard preference isn't the only preference option available for that call. I'd prefer to model it the same as we do with GetRequest, as in:

  def preference(pref: Preference): GetRequest = preference(pref.value)
  def preference(pref: String): GetRequest = copy(preference = pref.some)

[flavienbert]

The modification is done.
I initially overlooked the existing model, but I’m not a big fan of it:
case class Shards(ids: List[String]) extends Preference("_shards:" + ids.mkString(",")).

It doesn’t enforce that at least one shard is provided or same shards are provided, and shards are defined as String instead of Int.
My version case class ShardsPreferenceRequest(shard: Int, optionalShards: Option[Set[Int]] = None) feels safer and more type-safe.