Table function implementation.

[lowka]

Provides implementation of a table function that does not store its state inside SQL row.

1. ScanNode now uses TableFunction instead of an iterable.
2. Every scan creates a new instance of table function, and then uses it to produce more rows.
3. When scan is complete that instance is closed.

TableFunction - some table function. TableFunctionInstance - concrete implementation of a table function (Scan operator creates an instance of table function per scan. TableFunctionProvider (for a lack of a better name) converts RexCalls into TableFunctions

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