compute: dropdb DROP SUBSCRIPTION fix (#10066)

## Problem
Project gets stuck if database with subscriptions was deleted via API /
UI.

https://github.com/neondatabase/cloud/issues/18646

## Summary of changes
Before dropping the database, drop all the subscriptions in it.
Do not drop slot on publisher, because we have no guarantee that the
slot still exists or that the publisher is reachable.

Add `DropSubscriptionsForDeletedDatabases` phase to run these operations
in all databases, we're about to delete.
Ignore the error if the database does not exist.

compute_tools/src/sql/drop_subscription_for_drop_dbs.sql

@@ -0,0 +1,11 @@
+DO $$
+DECLARE
+    subname TEXT;
+BEGIN
+    FOR subname IN SELECT pg_subscription.subname FROM pg_subscription WHERE subdbid = (SELECT oid FROM pg_database WHERE datname = {datname_str}) LOOP
+        EXECUTE format('ALTER SUBSCRIPTION %I DISABLE;', subname);
+        EXECUTE format('ALTER SUBSCRIPTION %I SET (slot_name = NONE);', subname);
+        EXECUTE format('DROP SUBSCRIPTION %I;', subname);
+    END LOOP;
+END;
+$$;