[DPE-5987] Add expose-external configuration option

[Batalex]

This PR ports the changes of canonical/zookeeper-k8s-operator#116.

External access by setting up a service is specific to K8s charms, but this PR brings the following:

• Configuration: the charm configuration is now validated using pydantic, similar to what we have on Kafka.
• Charm lib: the ZooKeeper charm lib was looking for the ZK leader unit upon instantiating ZooKeeperManager. This behaviour is incompatible with an application-wide nodeport service that can route queries to any unit. The default behaviour is to look for any live ZK unit. We will need to publish this change before any client Charm can relate to ZK with expose-external set to something else than "false"
• Relation interface: the endpoints relation field now properly concats the port to each ZK host

[deusebio]

Looks very good! Thanks for backporting this!