feat(log): add nested log format support for logger plugins

[TaeyeongKwak]

Description

This PR adds support for nested log format in logger plugins. Previously, the log_format configuration only supported flat key-value structures. This feature allows users to more efficiently organize and group log fields by defining hierarchical/overlapping log structures.

For example, user can set log_format as below:

{
    "log_format": {
        "host": "$host",
        "client_ip": "$remote_addr",
        "request": {
            "method": "$request_method",
            "uri": "$request_uri",
            "headers": {
                "user_agent": "$http_user_agent"
            }
        },
        "response": {
            "status": "$status"
        }
    }
}

Output Example:

{
    "response": {
        "status": 200
    },
    "route_id": "1",
    "client_ip": "127.0.0.1",
    "request": {
        "method": "GET",
        "uri": "/",
        "headers": {
            "user_agent": "curl/8.5.0"
        }
    },
    "host": "127.0.0.1"
}

Which issue(s) this PR fixes:

Fixes #

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible (If not, please discuss on the APISIX mailing list first)