Add ldap storage

[nblock]

The ldap backend is currently limited to a few attributes (mostly phone related) but I'm willing to expand this to other vcard fields. It is currently being tested with an OpenLDAP server and supports the inetOrgPerson schema. In my test setup I'm syncing from Nextcloud to OpenLDAP and from OpenLDAP to filesystem. This PR is not complete yet, but I'm looking for feedback.

Tasks:

• List entries in LDAP
• Get an entry from LDAP
• Create new entry in LDAP
• Update an existing entry in LDAP
• Delete an entry from LDAP
• Support metadata?
• Support collections?
• Fix CI builds
• Fix tests
• Documentation
• Cleanup, squash and proper commit message

This is more or less a continuation of #500 by @paulfariello (thanks for your effort!).

See also: #500
Fixes: #283

[WhyNotHugo]

I'm not very familiar with LDAP, but don't see anything obviously wrong here.

I'm curious about the LDAP<>vcard conversion that you're doing. Some fields in vcard don't seem to be supported in LDAP. This means that when you sync vcard->ldap the fields are dropped, but if things later change on the LDAP side and the contact is synced ldap->vcard those dropped fields will be lost forever, right?