1.43.0 - Leverage the new support for nested path expressions in SQLKit

This patch was authored and released by @gwynne.

Fluent now takes generic advantage of the new support added by vapor/sql-kit#169 when generating nested subpath expressions.

(Unfortunately, Fluent has never actually invoked this code path to begin with, except via an explicit call to .sql(json:_:), so it doesn't change much at the moment. However, this would always have been a prerequisite to make the real support for it in Fluent's API actually work, so it's a step in the right direction.)

SQLJSONColumnPath is now deprecated in favor of using SQLKit's new SQLNestedSubpathExpression. #572