Add stats API to Direct Transfer

What does this MR do and why?

This MR adds stats to Direct Transfer API so we can see how many objects were fetched & imported. Similar to GitHub Importer stats we provide.

Migration output

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main: == [advisory_lock_connection] object_id: 174460, pg_backend_pid: 62754
main: == 20231016092554 AddObjectCountFieldsToBulkImportTrackers: migrating =========
main: -- add_column(:bulk_import_trackers, :imported_objects_count, :integer, {:null=>false, :default=>0})
main:    -> 0.0014s
main: -- add_column(:bulk_import_trackers, :fetched_objects_count, :integer, {:null=>false, :default=>0})
main:    -> 0.0005s
main: == 20231016092554 AddObjectCountFieldsToBulkImportTrackers: migrated (0.0054s) 

main: == [advisory_lock_connection] object_id: 174460, pg_backend_pid: 62754
ci: == [advisory_lock_connection] object_id: 174760, pg_backend_pid: 62756
ci: == 20231016092554 AddObjectCountFieldsToBulkImportTrackers: migrating =========
ci: -- add_column(:bulk_import_trackers, :imported_objects_count, :integer, {:null=>false, :default=>0})
ci:    -> 0.0011s
ci: -- add_column(:bulk_import_trackers, :fetched_objects_count, :integer, {:null=>false, :default=>0})
ci:    -> 0.0005s
ci: == 20231016092554 AddObjectCountFieldsToBulkImportTrackers: migrated (0.0084s) 

update! query plan & execution

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 EXPLAIN UPDATE "bulk_import_trackers" SET "updated_at" = '2023-10-19 08:54:04.889990', "fetched_objects_count" = 1 WHERE "bulk_import_trackers"."id" = 956;
                                                 QUERY PLAN                                                  
-------------------------------------------------------------------------------------------------------------
 Update on bulk_import_trackers  (cost=0.14..2.16 rows=0 width=0)
   ->  Index Scan using bulk_import_trackers_pkey on bulk_import_trackers  (cost=0.14..2.16 rows=1 width=18)
         Index Cond: (id = 956)
(3 rows)

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Before	After

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