client: Introduce --sentry-nodes flag (#4285)

* client/authority-discovery: Add smoke tests for intervall_at

* client/authority-discovery: Fix interval_at to fire on start

* .maintain/sentry-node: Update docker compose file

* client: Introduce --sentry-nodes flag

Enable operators to specify the public addresses of sentry nodes infront
of a validator node so that the validator node can announce the sentry
node addresses instead of its own public addresses on the DHT via the
authority discovery module.

* client/authority-discovery: Break lines at 100 characters

Limit line length to 100 instead of 120 characters.