Collectd input should support encryption/signing

A collectd input got merged in recently (PR #785). The collectd protocol has the possiblity to sign and encrypt the data it sends (binary protocol description, small description for configuration, discussion/description on mailinglist, Collectd source).

It would be awesome if logstash supports this as well.

Additional options for this plugin would be needed:

• security_level: to specify the security level, one of none, sign, encrypt defaults to none.
• authentication_file: to specify the path to the authentication file on-disk, mandatory if security_level is sign or encrypt.

New stuff needed:

• Add function to generate the shared secret
• Add function to decrypt a package
• Add function to validate signature

The authentication_file should be monitored for changes (inotify?) so no restart is needed to have new collectd's send logs to logstash.