user_external External user support

Use external user authentication methods like IMAP, SMB and FTP

Use external user authentication methods like IMAP, SMB and FTP tools integration AGPL Robin Appelman https://docs.nextcloud.com/server/15/admin_manual/configuration_user/user_auth_ftp_smb_imap.html https://github.com/nextcloud/user_external/ https://github.com/nextcloud/user_external/issues https://github.com/nextcloud/user_external.git 0.5