Logstash snmp output plugin

There is already an snmp input but there are also scenarios where an snmp output plugin could be useful. E.g. sending messages from logstash to another tool.

As snmp is a very common protocol there are many use cases this can come in handy.

Start by reading the existing SNMP input and the project's output-plugin conventions. The issue proposes sending Logstash messages to another tool over SNMP, but it does not name files, tests, supported operations, or acceptance criteria; clarify those requirements before implementation.

Written by the indexing model from the issue text.