chore: unify UPSERT implementation

[SamuelBrucksch]

This PR unifies the UPSERT implementation of all the database services. resulting in 2 helper functions in each database service that allow for defining the database specific JSON extraction logic and the database specific undefined check. With these special cases covered it is possible to generate INSERT and UPSERT statements that correctly apply default, @cds.on.insert and @cds.on.update values. Which currently only apply to @cap-js/hana and is a big blocker for most use cases of UPSERT and the @sap/cds/common.cds use the @cds.on.insert annotations. Which with the current UPSERT implementation could possibly be null which should be impossible as every entry in the database has to have been inserted once.