Review code that downcast_ref from CatalogSession to SessionState

[findepi]

#11516 added downcast_ref to SessionState in couple places with TODO comments. These places should be revised. Perhaps CatalogSession API should be extended so that downcast is not needed, or functionality refactored (e.g. as proposed here: #11516 (comment)).

Context

• Extract catalog API to separate crate, change TableProvider::scan to take a trait rather than SessionState #11516