Make "docker buildx inspect" return a json string like other inspect commands in docker

Same reasons as https://github.com/docker/buildx/issues/380 , for a third party lib, it's much easier to work with.

$ docker network inspect ffb3f184cd0a
[
    {
        "Name": "host",
        "Id": "ffb3f184cd0a2077f75a507320a9613eec135dc6cb234340ea924d576215e96e",
        "Created": "2020-05-04T17:50:40.0997657+02:00",
        "Scope": "local",
        "Driver": "host",
        "EnableIPv6": false,
        "IPAM": {
            "Driver": "default",
            "Options": null,
            "Config": []
        },
        "Internal": false,
        "Attachable": false,
        "Ingress": false,
        "ConfigFrom": {
            "Network": ""
        },
        "ConfigOnly": false,
        "Containers": {
            "a8d13ad9ac75a3343da098003e22052ac6ada63fa03a216e96f549f636b5ab56": {
                "Name": "gabriel_work_env",
                "EndpointID": "fe22d2b01c99eca9992079cfe1bc04774d9f1e29c4c161808273a286cc018ae7",
                "MacAddress": "",
                "IPv4Address": "",
                "IPv6Address": ""
            }
        },
        "Options": {},
        "Labels": {}
    }
]

$ docker buildx inspect blissful_swartz
Name:   blissful_swartz
Driver: docker-container

Nodes:
Name:      blissful_swartz0
Endpoint:  unix:///var/run/docker.sock
Status:    inactive
Platforms:

Start at the entry point for docker buildx inspect and compare its current output with the JSON returned by docker network inspect. Determine the JSON shape needed for buildx inspection data; done means the command returns machine-readable JSON suitable for third-party libraries, while preserving the existing inspection information.