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.

cmd/go_migrate/main.go

@@ -118,7 +118,7 @@ func processMigrationJobs(
 
 	sourceTableAnalyzer := table_analyzers.NewMssqlTableAnalyzer(sourceDb)
 	targetTableAnalyzer := table_analyzers.NewPostgresTableAnalyzer(targetDb)
-	extractor := extractors.NewMssqlExtractor(sourceDb)
+	extractor := extractors.NewExtractor(sourceDb)
 	loader := loaders.NewGenericLoader(targetDb)
 
 	var azureClient *azure.Client

cmd/go_migrate/process.go

@@ -39,7 +39,7 @@ func processMigrationJob(
 	targetDbWrapper dbwrapper.DbWrapper,
 	sourceTableAnalyzer etl.TableAnalyzer,
 	targetTableAnalyzer etl.TableAnalyzer,
-	extractor etl.Extractor,
+	extractor extractors.GenericExtractor,
 	azureClient *azure.Client,
 	loader etl.Loader,
 	job config.Job,
@@ -155,9 +155,8 @@ func processMigrationJob(
 
 	for range maxExtractors {
 		wgExtractors.Go(func() {
-			extractors.Consume(
+			extractor.Consume(
 				localCtx,
-				extractor,
 				job.SourceTable,
 				sourceColTypes,
 				job.BatchSize,