Add SNMPv3 support to snmptrap

[edmocosta]

• Added SNMPv3 support to logstash-input-snmptrap
• All SNMPv3 credentials settings and validations were moved from the snmp plugin to the common mixin.
• Added the local_engine_id option for the snmp input plugin
• Adjusted a few options descriptions order (docs).
• Changed MIB manager to read files sorted by path, so lookups based on the module name (RubySnmpOidFieldMapper for example) are deterministic.

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SNMPv3 tests will be added through: https://github.com/logstash-plugins/logstash-integration-snmp/pull/9