Add support for arbitrary RFC 5424 structured data via options.struct…

[wesgarland]

This patch adds RFC 5424 structured data to the module.

• specify structured data with options.structuredData
• specifying this option forces options.rfc3164 = false
• the format is

        options.structuredData = {
          id1: { param1: value1, param2: value2 },
          id2: { param1: value1, param2: value2 },
        }

Tested with BetterStack.com syslogd in both TLS and UDP.