Retrieve public SSH keys from LDAP.

Add new class `LdapPublicKeyManager` which retrieves public SSH keys
from LDAP.

The attribute can be configured with the new configuration option
`realm.ldap.sshPublicKey`. The setting can be a simple attribute name,
like `sshPublicKey`, or an attribute name and a prefix for the value,
like `altSecurityIdentities:SshKey`, in which case attributes are selected
that have the name `altSecurityIdentities` and whose values start with
`SshKey:`.