Add option 'modPassPlain' to send plaintext password updates to LDAP server

[spike77453]

This adds an additional option modPassPlain to the authldap plugin. Sending a password update to an LDAP server in plaintext is useful is a few cases:

• The hashing algorithm and/or format is not available in dokuwiki (e.g. RedHat's pbkdf2_sha256 format, the default for 389ds and RHDS)
• A password policy is enabled on the server. This can't be enforced when sending hashed passwords
• Sending hashed passwords is disabled or only available to privileged accounts (password managers, directory managers) on the LDAP server (e.g. nsslapd-allow-hashed-passwords is off per default on 389ds and RHDS). Using plaintext passwords allows binding with a service account or as user with less privileges
• The number of rounds is (e.g. for pbkdf2_sha256) is either unknown or cumbersome to manage in dokuwiki (e.g. 389ds dynamically sets the number of rounds depending on hardware capabilities)

This obviously should only be used via a secure or private communication channel (ldaps, starttls, ldapi).