feat: update extractor and transformer constructors to return Extractor interface

internal/app/etl/extractor/mssql.go

@@ -21,7 +21,7 @@ type MssqlExtractor struct {
 	db *sql.DB
 }
 
-func NewMssqlExtractor(db *sql.DB) *MssqlExtractor {
+func NewMssqlExtractor(db *sql.DB) Extractor {
 	return &MssqlExtractor{db: db}
 }