consul service: Use Type=notify

[nh2]

Extracted from #49165 (comment):

• Set Type=notify on the systemd unit. Apparently this is supported.

When running under systemd on Linux, Consul notifies systemd by sending READY=1 to the $NOTIFY_SOCKET when a LAN join has completed. For this either the join or retry_join option has to be set and the service definition file has to have Type=notify set.
https://www.consul.io/docs/agent/basics.html#running-an-agent

This will improve detection on when consul is actually up.

CC @arianvp

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