in_syslog: add option 'original_message_key'

[candlerb]

(Note: this applies on top of PR #2499 )

Which issue(s) this PR fixes:

N/A

What this PR does / why we need it:

Some log systems like loki natively work on raw text lines, and are more natural to use in this form as opposed to a JSON collection of parsed fields.

Loki can accept certain parsed fields out-of-band as tags, but only if low-cardinality. syslog parsing can generate high cardinality values, in particular pid, so all these values cannot be mapped to tags. But if they are not mapped to tags, and the original line is not kept, then these values are lost.

I think it would also be useful with out_file to record the entire original syslog line.

Docs Changes:

Add new setting to https://github.com/fluent/fluentd-docs-gitbook/blob/1.0/plugins/input/syslog.md

Release Note:

### Enhancement

in_syslog: add `original_message_key` to retain the entire original message

[repeatedly]

This is done by meta parser like multi-format-parser and it is more general approach.

<parse>
  @type keep_original  # wrap other parser and assign original `text` into parsed result.
  <parser>
    @type json
  </parser>
</parse>

[candlerb]

For benefit of readers here is a link to multi-format-parser