Allow to configure the aggregation functions for default star tree index

[chrajeshbabu]

Currently in the default star tree pre-aggregating sum and count of the metrics. It would be better to allow configuring other functions like min, max in the table index configs. In our usecase most of the queries fits through the default star tree index but cannot use because other functions like min, max required in queries.

    TreeMap<AggregationFunctionColumnPair, AggregationSpec> aggregationSpecs = new TreeMap<>();
    aggregationSpecs.put(AggregationFunctionColumnPair.COUNT_STAR, AggregationSpec.DEFAULT);
    for (String numericMetric : numericMetrics) {
      aggregationSpecs.put(new AggregationFunctionColumnPair(AggregationFunctionType.SUM, numericMetric),
          AggregationSpec.DEFAULT);
    }