input: add support for extended metadata

[yaauie]

This is a Work-In-Progress solution for https://github.com/logstash-plugins/logstash-integration-rabbitmq/issues/13, in which a user has a need for the raw payload.

Since raw payload metadata drastically increases the size of in-flight events, I wanted to implement this in a way that would only incur that overhead for people who explicitly need raw payloads.

To do this, I changed the formerly boolean metadata_enabled to instead accept a value from a list:

• none: no metadata (default)
• basic: header and property metadata
• extended: header, property, and payload metadata
• false: deprecated alias for none
• true: deprecated alias for basic

Needs tests, but hoping for validation of concept before I dive in.

[andsel]

Hi @yaauie to me seems good, I would add just a couple of tests to avoid regression.

[andsel]

LGTM