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

[membphis]

What this PR does / why we need it:

as title.

For example:

curl http://****/apisix/admin/plugin_metadata/http-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?

[moonming]

why not add log_format in http-logger plugin directly?

[moonming]

Different routes may have different log formats

[moonming]

What is the difference between metadata and the properties of plugin itself? I didn't see why we need it.

[membphis]

get plugin schema:
/apisix/admin/plugins/{plugin-name}, eg: /apisix/admin/plugins/limit-req

get plugin metadata:
/apisix/admin/plugin_metadata/{plugin-name}, eg: /apisix/admin/plugin_metadata/example-plugin

[moonming]

#2294 (comment)

[moonming]

why we need this PR?

[juzhiyuan]

I would recommend using /plugins/{name}/metadata in a RESTful way if this PR is needed..

[ShiningRush]

I am also curious about what scenarios need metadata instead of plugin's option.

[membphis]

this PR has been merged. we can talk in here: #2307 (comment)