Use SQL functions instead of compute_stats UDAF to compute column statistics

Currently we compute column statistics by relying on the compute_stats UDAF. For instance, for a given table tbl, the query to compute statistics for columns is translated internally into:

SELECT compute_stats(c1),
       compute_stats(c2),
       ...
FROM tbl;

compute_stats produces data for the stats available for each column type, e.g., struct<"max":long,"min":long,"countnulls":long,...>.

This issue is to produce a query that relies purely on SQL functions instead:

SELECT max(c1), min(c1), count(case when c1 is null then 1 else null end),
       ...
FROM tbl;

This will allow us to deprecate the compute_stats UDAF since it mostly duplicates functionality found in those other functions. Additionally, many of those functions already provide a vectorized implementation so the approach can potentially improve the performance of column stats collection.