fix(isthmus): support expressions in Sort

[yassram]

Motivation

This PR adds support for expressions in Sort (today, only input references are supported #192).

Details

This PR uses Calcite's relbuilder.sort(rexNodes) rather than computing RelFieldCollation (which required input references only). Calcite then makes sure to expand the Sort into a LogicalSort and a LogicalProject to evaluate expressions if needed (an additional LogicalProject is applied which acts like a remap to remove any introduced fields to preserve the same output).

Examples:

e1:

SELECT foo FROM bar ORDER BY foo

LogicalSort(sort0=[$0], dir0=[ASC])
  TableScan(table=[bar])

e2:

SELECT foo FROM bar ORDER BY foo::int + 1 DESC NULLS LAST

LogicalProject(foo=[$0])
  LogicalSort(sort0=[$1], dir0=[DESC-nulls-last])
    LogicalProject(foo=[$0], $f1=[+(CAST($0):BIGINT, 1)])
      TableScan(table=[bar])

See ComplexSortTest.java for more examples

[CLAassistant]

All committers have signed the CLA.

[vbarua]

Overall looks good to me. It's good to be able to rely on built-in Calcite functionality to expand complex expressions in the sort fields.

I did leave one comment on your tests. The CollationRelWriter seems somewhat redundant given that that the same information is visible in the LogicalSorts

[vbarua]

Nice find! It's really helpful that we can rely on Calcite's machinery to do this for us.