fix(stream): let Row Merge executor handle extra row count column in input

[kwannoel]

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What's changed and what's your intention?

Row Count column will not have a corresponding mapping in the i2o map. So if we chance upon a column without a mapping, we can just ignore it.

Also added a test for approx percentile mixed with normal agg. Subsequent PR will also add support for stateful agg e.g. min/max.

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[chenzl25]

LGTM