Provide documentation for connection between Masterportal and Frost

As an admin I want to display data from Frost's SensorThingsApi in the Masterportal frontend.

This fails due to error 401, when Masterportal sends a get request to Frost. We don't see how the communication between Masterportal and Frost works, since Apisix evaluates tokens from the api-access client for Frost and tokens from the geostack client for Masterportal. However we don't even see a token in the request from Masterportal to Frost and wonder how the configuration needs to be done in order to establish a connection between those two services.

For our scenario we use accounts from Keycloak to login to Masterportal. They're members of the ds_intern group in Keycloak.

This is from our config.json:

{
  "Themenconfig": {
    "Hintergrundkarten": {
      "Layer": [
        {
          "isNeverVisibleInTree": false,
          "name": "TopPlus Open",
          "id": "3",
          "visibility": true
        }
      ]
    },
    "Fachdaten": {
      "Ordner": [
        {
          "Titel": "Wetter",
          "isFolderSelectable": true,
          "Layer": [
            {
              "id": "weather_service_ol",
              "visibility": true
            }
          ]
        }
      ]
    }
  }
}

And this is from our services.json:

[
  {
    "id": "weather_service_oldenburg",
    "name": "Weather Service - Oldenburg",
    "typ": "SensorThings",
    "version": "1.1",
    "url": "https://api.civitas.mycompany.com/context/sensorthings/v1.1",
    "intersect": true,
    "urlParameter": {
      "root": "Things",
      "filter": "startswith(Things/name,'Weather Station')",
      "expand": [
        "Locations",
        "Datastreams/Observations($orderby=phenomenonTime%20desc;$top=1)",
        "Datastreams/ObservedProperty",
        "Datastreams/Observations"
      ]
    },
    "roles": [
      "ds_intern"
    ]
  }
]