Add ability to configure external DCS

Hello, everyone!

It would be great to allow configuring external DCS like [consul](https://patroni.readthedocs.io/en/latest/ENVIRONMENT.html#consul), etcd or [zookeeper](https://patroni.readthedocs.io/en/latest/ENVIRONMENT.html#zookeeper) for PostgreSQL charm .

This feature may be really useful for the following reasons:
1) Users can have a more flexible and reliable setup in their production environment. E.g. let's imagine the case when the user already has a reliable Consul or Zookeeper N-node cluster and they could just connect the charm to it and have reliable PostgreSQL
2) Users think that it's too dangerous to use the deprecated feature(raft support in Patroni) in production env
3) It seems that bugs like https://github.com/canonical/postgresql-operator/issues/570 or https://github.com/canonical/postgresql-operator/issues/573 may be work-arounded via external DCS