SQL: Add rule for merging nested Aggregates.

[gianm]

The rule is adapted from Calcite's AggregateMergeRule, with two changes:

1. Includes a workaround for https://issues.apache.org/jira/browse/CALCITE-7162

2. Includes the ability to merge two Aggregate with a Project between them, by pushing the Project below the new merged Aggregate.

The goal is to rewrite a query like:

SELECT
  hr,
  UPPER(t1.x) x,
  SUM(t1.cnt) cnt,
  MIN(t1.mn) mn,
  MAX(t1.mx) mx
FROM (
  SELECT
    floor(__time to hour) hr,
    dim2 x,
    COUNT(*) cnt,
    MIN(m1 * 5) mn,
    MAX(m1 + m2) mx
  FROM druid.foo
  WHERE dim2 IN ('abc', 'def', 'a', 'b', '')
  GROUP BY 1, 2
) t1
WHERE t1.x IN ('abc', 'foo', 'bar', 'a', '')
GROUP BY 1, 2

Into:

SELECT
  FLOOR(__time TO hour) hr,
  UPPER(dim2) x,
  COUNT(*) cnt,
  MIN(m1 * 5) mn,
  MAX(m1 + m2) mx
FROM druid.foo
WHERE dim2 IN ('abc', 'a', '')
GROUP BY 1, 2

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