Identify Group by columns

[Laukkala]

resolves #826

Description

This PR adds metadata to outputs of ChartStep, TimeChartStep, StatsStep and EventStatsStep which can be used to identify the columns that were used in a groupBy operation when executing the step.
This information can be accessed by obtaining the Dataset's schema, and then checking for the existence of a key called "dpl_internal_isGroupByColumn".

Metadata is used instead of inspecting the LogicalPlan of the resulting dataset, because Dataset.writeStream().forEachBatch() overrides the LogicalPlan for each batch. Metadata, on the other hand, does not get overwritten.

This functionality is required for zep_01#283, where data formatting needs to get access to column names that were used for grouping data.

Checklists

Testing

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