feat(kafka-logger): support for specified the log formats via admin API.

[chzhuo]

What this PR does / why we need it:

as title.

For example:

curl http://****/apisix/admin/plugin_metadata/kafka-logger -d '
{
    "log_format": {
        "host": "$host",
        "@timestamp": "$time_iso8601",
        "client_ip": "$remote_addr"
    }
}'

when we enabled plugin http-logger, we will get the message body like:

{"host":"localhost","@timestamp":"2020-09-23T18:29:07-04:00","client_ip":"127.0.0.1","route_id":"1"}
{"host":"localhost","@timestamp":"2020-09-23T18:29:07-04:00","client_ip":"127.0.0.1","route_id":"1"}

Pre-submission checklist:

• Did you explain what problem does this PR solve? Or what new features have been added?
• Have you added corresponding test cases?
• Have you modified the corresponding document?
• Is this PR backward compatible? If it is not backward compatible, please discuss on the mailing list first