[DF] Generalize CREATE | PREDICT MODEL to accept non-native SELECT statements

[charlesbluca]

Generalizes CREATE | PREDICT MODEL to accept DaskStatements instead of SQLStatements, so that we can do things like:

SELECT * FROM PREDICT (
    MODEL my_model,
    SELECT x, y FROM PREDICT (
        ...
    )
)

This is something that would've happened in working on FILTER WHERE support, but figured it might make sense to break this out into its own PR.

[codecov-commenter]

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