Add support for logstash codecs

[ToToL]

Hi !

Add support for executing codec for body of http request.

I don't know why it has not been done before, maybe there is a reason. Let me know if it is.

Regards

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[kares]

👍 this is a sold start but in order to ship the feature needs more work:

• deprecate the format option in favor of the codec setting
• format => codec looks like a hack - would keep format as is (just mark it obsolete)
• format default (:default => 'json') should be removed
• plugin should detect a format configured by user (!= nil) and use it
• if both codec and format are user configured the plugin should error
  (this is a bit tricky - happy to assist as/if needed when other pieces are done)
• the new version should be a minor bump

(sorry for the late review)