bug: can't handle aggregates within other value operations

[gforsyth]

What happened?

We're handling certain aggregate functions incorrectly.

If the aggregate is within a separate ValueOp, like the below somecol.sum() / sumcol.sum() then we hit

TypeError: Parameter to MergeFrom() must be instance of same class: expected substrait.ibis.Expression got substrait.ibis.AggregateFunction.

because we're recursing down into the expression, but the AggregateRel needs to be handled outside of the current expression (i'm not exactly sure how at this point)

Here's a failing test case:

def test_value_op_aggregate(compiler):
    t = ibis.table([("a", "float"), ("b", "int32")], name="t")
    expr = t.aggregate(mkt_share=t.a.sum() / t.b.sum())

    result = translate(expr, compiler)

    assert result

What version of ibis-substrait are you using?

2.21.0

What substrait consumer(s) are you using, if any?

No response

Relevant log output

No response

[gforsyth]

The first part of this to handle is the Value dispatch when either/both of left and right are a reduction e.g. t.a.sum() / 2 or t.a.sum() / t.b.sum()

The topological sort of:

    t = ibis.table([("a", "float"), ("b", "int32")], name="t")
    expr = t.aggregate(mkt_share=t.a.sum() / 2 )

will be something like
table -> literal -> col -> sum -> divide -> alias -> aggregate

but we currently fail at divide because we are treating the sum as an input to a "regular" scalar function.

We instead want to translate that into (roughly):

    # The Literal -> Col -> Sum -> Divide should resolve to:
    # project:
    #   common:
    #     emit: ...
    #   input:
    #     aggregate:
    #        input: table
    #        measures: sum
    #   expressions:
    #     scalarFunction:
    #     arguments:
    #       value:
    #         selection:
    #           directReference:
    #             structField
    #               ...
    #       value:
    #         literal:
    #           ...