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ARROW-6952: [C++][Dataset] Implement predicate pushdown with ParqueFileFragment

The proposed change can be divided in 3 parts:

- Implement the `StatisticsAsScalars(Statistics& stats, Scalar* min, Scalar* max)`
  function to convert `parquet::Statistic`s min and max as `arrow::Scalar`s.

- Implement the `RowGroupStatisticsAsExpression(RowGroupMetadata& meta, Expression* out)`
  function to represents the RowGroup's statistics as an expression of
  conjunction, e.g. `(a_min <= a AND a <= a_max) AND (b_min <= b AND b <= b_max) AND ...`

- Modifies ParquetScanTaskIterator to skip RowGroups by checking the
  expression derived from the metadata with the filter expression.

Closes #5765 from fsaintjacques/ARROW-6952-dataset-parquet-predicate-pushdown and squashes the following commits:

5fd0efd6c <François Saint-Jacques> Add unit test and fix issues
04e4a45a3 <François Saint-Jacques> Review comments
0d14b8f58 <François Saint-Jacques> Expose SchemaManifest publicly
235a0d699 <François Saint-Jacques> ARROW-6952:  Implement predicate pushdown with ParquetFileFragment

Authored-by: François Saint-Jacques <fsaintjacques@gmail.com>
Signed-off-by: Neal Richardson <neal.p.richardson@gmail.com>