Support projection post operator

[navis]

To retain only required columns in result. We can make columns via post-aggregator, etc. but cannot remove column which is not needed or should not be returned.

[navis]

example:

"context": {
   "postProcessing" : {
      "type": "projection",
      "columns": ["column", "list", "you", "want", "to", "retain"]
   }
}

[gianm]

My feeling is that this is a neat feature but would better belong in the ToolChests (probably in postMergeQueryDecoration) than in a post processing operator. That way it would be extensible for custom queries.

[navis]

@gianm Thought about that but for that we need add new fields (something like "retainColumns") in query objects. would it be better?

[gianm]

IMO it wouldn't be too bad to add an optional "projection" field to any query that wants to support projections. welcome opinions from others though.

[navis]

@gianm addressed comment. added outputColumns to timeseries/groupby/topn/select queries.

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