feat: initial version of the ldap interface

[wood-push-melon]

This pull request tries to present the preliminary version of the juju charm library for the ldap juju interface. This version implements fundamental provider, consumer, and juju custom events.

Note that this juju charm library is subject to change once the design of the glauth-k8s charm is finalized.

[wood-push-melon]

As we discussed, we need to implement a update_app_data (or something along those lines) method in LdapProvider, that will be used by the charm to update the ldap_uri and base_dnin all relations.

We can merge it without that, but please create an issue to keep track of it.

Yep, will convert my local TODO list to some JIRA tickets for the memo.