feat(core): add ingress server mode

[bzp2010]

Description

Add server mode to the ADC that exposes the HTTP API, which will help the ingress controller reduce launch and compile overhead.

This should not be used by any internal or external scenario other than a non-ingress controller unless you fully understand the ADC and know what you are doing.

It accepts the following request formats:

PUT /sync

{
    "task": {
        "opts": {
            "backend": "apisix",
            "server": "http://127.0.0.1:9180",
            "token": "token"
        },
        "config": {
            "services": [],
            "routes": [],
            "...": []
        }
    }
}

Start it with adc server.

Please pay special attention to its access control. Right now, this API does not have any additional authentication or permission control mechanisms, so it should not be called from anywhere but the local host.
Now, this server always listens on 127.0.0.1 instead of 0.0.0.0. In the future, we may add security mechanisms such as mTLS, etc., which is up for discussion.

Since the main use of this feature is for the ingress controller, it is expected that we can simply implement mTLS through components such as cert-manager, but mechanisms such as API key or basic auth are also possible.

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

[juzhiyuan]

Other changes LGTM