refactor: replace per-dialect extractors with generic extractor

Collapse the mssql- and postgres-specific extractors into a single generic implementation driven by the DB dialect interface.

internal/app/db-wrapper/db_dialects/main.go

@@ -0,0 +1,7 @@
+package db_dialects
+
+const (
+	SqlServer string = "sqlserver"
+	Postgres  string = "postgres"
+	Null      string = "null"
+)