Define delayed event ratelimit category (#18019)

Apply ratelimiting on delayed event management separately from messages.

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---------

Co-authored-by: Andrew Morgan <1342360+anoadragon453@users.noreply.github.com>

docker/complement/conf/workers-shared-extra.yaml.j2

@@ -94,6 +94,10 @@ rc_presence:
     per_second: 9999
     burst_count: 9999
 
+rc_delayed_event_mgmt:
+  per_second: 9999
+  burst_count: 9999
+
 federation_rr_transactions_per_room_per_second: 9999
 
 allow_device_name_lookup_over_federation: true