Support Union in Decoupled planning

[kgyrtkirk]

• introduces UnionQuery
• some changes to enable multiple input datasources for a Query
• UnionQuery execution is driven by the QueryExecutor - which could later enable to reduce some complexity in ClientQuerySegmentWalker
• to service UnionQueryToolChest methods properly there was a need to get access to other ToolChest-s ; most of the related test system refactor is done in #17331
• renamed UnionQueryRunner to UnionDataSourceQueryRunner
• QueryRunnerFactoryConglomerate could act as a QueryToolChestWarehouse which shaves of some unnecessary things here and there
• PR contains https://github.com/apache/druid/pull/17331