Add "format" parameter to UDP input plugin.

Format parameter can be used to indicate how to parse the input data.
Allowed values are:
- plain: Do not try to parse the message received, provide the payload
    as `@message` attribute and fill the other attributes as before.
- json_event/(undefined) (default): Try to parse the payload as JSON,
    or fallback to plain text (as previous behavior).
- json: Try to parse the payload as JSON, filling the `@fields`
    attribute, or fallback to plain text (logstash-like).
- json_syslog: Try to parse the payload, starting at first '{', and
    filling the `@fields` attribute, or fallback to plain text
    (compatible with prefixed JSON message, such as for rfc3164 syslog).