Add worker tagging (#862)

This allows workers to have tags assigned to them, which can then be filtered to control which targets they are allowed to serve.

internal/db/schema/migrations/postgres/0/41_targets.up.sql

@@ -154,6 +154,7 @@ before insert on target_tcp
 
 
 -- target_all_subtypes is a union of all target subtypes 
+-- NOTE: this is replaced in 100 to add worker_filter
 create view target_all_subtypes
 as 
 select