Support indexof array expression function

[jonmmease]

The Vega indexof expression function is generated by the Vega-Lite oneOf predicate, so it would be great to support it in VegaFusion.

The exact expression form that Vega-Lite generates is:

indexof(["v1", "v2"], datum["col"]) !== -1)

I don't think indexof is a very common relational database expression function, but this particular expression could be transformed into col in ['v1', 'v2']. Initially, let's add a DataFusion UDF for indexof, but down the road we may want to add a specific expression transformation so that it's easier to compile this use of indexof into general SQL.