perf: Pushdown filter(False) to pyiceberg

[kevinjqliu]

Closes apache/iceberg-python#2681

Converts top-level true/false to pyiceberg's AlwaysTrue / AlwaysFalse. PyIceberg's scan function expects AlwaysTrue/AlwaysFalse (as BooleanExpression) instead of true/false boolean.

For cases such as

    pl.scan_iceberg(table, storage_options=storage_options)
    .filter(pl.lit(True))
    .explain()

    pl.scan_iceberg(table, storage_options=storage_options)
    .filter(pl.lit(False))
    .explain()

[kevinjqliu]

im having trouble with the test_scan_iceberg_filter_on_true_false test case. scan_iceberg().filter(pl.lit(False)) is not returning 0 records

[kevinjqliu]

pyiceberg's parser handles true/false -> AlwaysTrue/AlwaysFalse
https://github.com/apache/iceberg-python/blob/65ba59519170fa795bb7755d73910861c72b17c7/pyiceberg/expressions/parser.py#L283-L305