Possibility to retrieve systems grouped by operator

[hbruch]

Is your feature request related to a problem? Please describe.
Currently, neither gbfs/ endpoint (for GBFSv2) nor the manifest.json endpoint for GBFSv3 provide information, which systems are operated by which operator.

Goal / high level use-case
Mobility Apps usually group different systems by provider, e.g. to limit the number of options in preferences (see this discussion in the MobilityData GBFS slack).

Describe the solution you'd like
I suggest a new endpoint gbfs/operators.json which lists the operators (their id and name) and their published systems.

Describe alternatives you've considered
Alternatively,

• manifest.json could be extended to provide operator(s) information.
• the GraphQL API could be extended to provide links to the systems

Additional context
Example of a possible endpoint response:

{
    "operators": [
        {
            "id": "DER:Operator:deer",
            "name": {
                "translation": [
                    {
                        "language": "de",
                        "value": "Deer"
                    }
                ]
            },
            "systems": [
                {
                    "id": "deer",
                    "url": "http://localhost:8080/gbfs/deer/gbfs"
                }
            ]
        },
        {
            "id": "DKY:Operator:donkey",
            "name": {
                "translation": [
                    {
                        "language": "de",
                        "value": "Donkey Republic"
                    }
                ]
            },
            "systems": [
                {
                    "id": "donkey_ge",
                    "url": "http://localhost:8080/gbfs/donkey_ge/gbfs"
                }
            ]
        },
        {
            "id": "NBK:Operator:nextbike",
            "name": {
                "translation": [
                    {
                        "language": "de",
                        "value": "Nextbike"
                    }
                ]
            },
            "systems": [
                {
                    "id": "nextbike_df",
                    "url": "http://localhost:8080/gbfs/nextbike_df/gbfs"
                },
                {
                    "id": "nextbike_fg",
                    "url": "http://localhost:8080/gbfs/nextbike_fg/gbfs"
                },
                {
                    "id": "nextbike_ds",
                    "url": "http://localhost:8080/gbfs/nextbike_ds/gbfs"
                },
                {
                    "id": "nextbike_vn",
                    "url": "http://localhost:8080/gbfs/nextbike_vn/gbfs"
                }
            ]
        },
        {
            "id": "MEC:Operator:my-e-car",
            "name": {
                "translation": [
                    {
                        "language": "de",
                        "value": "My-E-Car"
                    }
                ]
            },
            "systems": [
                {
                    "id": "my-e-car",
                    "url": "http://localhost:8080/gbfs/my-e-car/gbfs"
                }
            ]
        }
    ]
}

[testower]

Complementary to #355