Support Value-based InSource

[alecgrieser]

Within the Cacades planner, there are methods for converting certain EXPLODE expressions into InSources for IN-join and IN-union planning. However, this logic is restricted to only LiteralValues (which get turned into InValues sources) and QuantifiedObjectValues (which get turned into InParameter sources). We're missing a few types, such as ConstantObjectValues.

We could add a Value-based InSource that can wrap ConstantObjectValues (etc) and then use those. There's a bit of work to consider here, to make sure that we don't use these values where it isn't appropriate. There's also some thought that we'll probably want to replace those with nested-loop joins and merge joins in the future, at which point we can probably remove these types of plans.