Amavis extension module to use Rspamd as a spam checker

       Copyright (c) 2019 Ralph Seichter, partially based on the SpamdClient extension. Released
       under GNU General Public License; see Amavis LICENSE file for details.

   Example configuration #1 (local Rspamd)
         # Rspamd running on the same machine as Amavis. Default URL
         # is http://127.0.0.1:11333/checkv2 , matching Rspamd's
         # "normal" worker defaults.
         @spam_scanners = ( [
             'Local Rspamd', 'Amavis::SpamControl::RspamdClient',

             # Adjust scores according to Rspamd's "required score"
             # setting (defaults to 15). Scores reported by Rspamd
             # will be multiplied with this factor. The following
             # adjusts Rspamd scores to SpamAssassin scores. While
             # this setting is technically optional, not adjusting
             # scores is prone to cause headaches.
             score_factor => $sa_tag2_level_deflt / 15.0,

             # MTA name is used to assess validity of existing
             # Authentication-Results headers, e.g. if DKIM/DMARC
             # validation has already happened.
             mta_name => 'mail.example.com',
         ] );

   Example configuration #2 (remote Rspamd)
         # Rspamd running behind HTTPS-capable proxy using basic
         # authentication to control access.
         @spam_scanners = ( [
             'Remote Rspamd', 'Amavis::SpamControl::RspamdClient',
             url => 'https://rspamd-proxy.example.com/checkv2',

             # Response timeout in seconds. Default is 60, matching
             # Rspamd's standard config for the "normal" worker.
             timeout => 42,

             # SSL-options and -credentials passed to LWP::UserAgent,
             # see https://metacpan.org/pod/LWP::UserAgent . Default:
             # ssl_opts => { verify_hostname => 1 },
             credentials => {
                 # The following <host>:<port> must match the 'url'
                 # defined above or credentials won't be transmitted.
                 netloc => 'rspamd-proxy.example.com:443',
                 # Remote authentication realm
                 realm => 'Rspamd restricted access',
                 username => 'Marco',
                 password => 'Polo',
             },

             # Don't scan messages remotely if the body size extends
             # the following limit (optional setting).
             mail_body_size_limit => 32 * 1024,

             score_factor => $sa_tag2_level_deflt / 15.0,
             mta_name => 'mail.example.com',
         ] );

   Requirements
       In addition to Amavis' core requirements, this extension needs the following additional
       Perl modules:

         JSON
         HTTP::Message
         LWP::UserAgent
         LWP::Protocol::https
         Net::SSLeay

       Should your host OS not provide the necessary packages, these modules can be obtained via
       https://www.cpan.org .