Support ExprNamed arguments to Scalar UDFs

[mach-kernel]

When encountering FunctionArg::ExprNamed nodes with "pg-style" function arguments, if the logical expression output is a literal, decorate its name/ident value to FieldMetadata.

Also has the side effect of supporting ExprNamed as Named is currently supported (I don't see why it shouldn't). I didn't see a way to do this with ExprPlanner.

Adding a TODO on our board to revert this patch once this ships:
apache#17379

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Gives us the ability to support RRF with named boosting subquery arguments:

SELECT id, title, fused_score
FROM rrf(
    vector_search(wiki_a_potion, 'machine learning algorithms', rank_boost => 10),
    text_search(wiki_a_potion, 'neural networks deep learning')
 );

Gives us the ability to provide named arguments to RRF:

SELECT id, title, fused_score
FROM rrf(
    vector_search(wiki_a_potion, 'machine learning algorithms', rank_boost => 10),
    text_search(wiki_a_potion, 'neural networks deep learning'),
    recency_decay_algorithm => 'exponential'
 );

...

[Column(Column { relation: None, name: "wiki_a_potion" }), Literal(Utf8("machine learning algorithms"), None), Literal(Int64(10), Some(FieldMetadata { inner: {"spice.parameter_name": "rank_boost"} }))]