Add support for LISTOF function, array return, and EXPLODE operator in PyDough

[knassre-bodo]

Linked ticket

Closes #521
Closes #522

Type of change

• Bug fix
• New feature
• Refactor
• Docs / config

What changed and why?

Adds the LISTOF function to PyDough to aggregate data into arrays, and the EXPLODE operator to flatten such arrays into multiple rows of table data. Also added support for DataFrame collections containing arrays, which become array literals in the SQL.

• All of these features are not supported for ANSI or SQLITE because those dialects do not have those capabilities.
• Array literals are not supported for Snowflake because the dialect does not have that capability.
• EXPLODE is not yet supported currently for MySQL or Oracle due to additional complications with those dialects.

How I tested this?

• Multiple new tests on TPCH data with the name prefix array_data_ to test the LISTOF and array literal features.
• Multiple new tests on TPCH data with the name prefix explode_ to test different variations of the EXPLODE operator, both on arrays and strings. Variations include:
  • Regular collection.EXPLODE(data)
  • Inside of a child context collection.CALCULATE(x=COUNT(EXPLODE(data)))
  • Exploding array data vs string data
  • Different cardinality/distinctness information about the exploded data
  • Exploded data being used for grouping and for window operations
  • Nested explosions (e.g. splitting an already split string with collection.EXPLODE(...).EXPLODE(...))
  • Splitting strings on an empty delimiter
  • DataFrame tables with varying data-types, empty arrays, nulls
• Error tests for incorrect uses of the EXPLODE operator (e.g. bad arguments)

Notes for reviewers

• A few changes to how the database connections are handled to account for difference in how the connectors return arrays.
• Several of the tests have dialect-specific skip marks. Some of these are truly unsupported features, and some are just for now unsupported.

[knassre-bodo]

This is not a thing that should be allowed with unqualified nodes, since any method or property name you add becomes something that can no longer be used as a PyDough term name.