JWT placeholder resolving does not correctly expand nested arrays

[thjaeckle]

Question #1984 brought up an issue with the jwt placeholder.

Given the following JWT:

{
  "authorization": {
    "permissions": [
      {
        "scopes": [ "device.read" ],
        "resource_set_id": "123456"
      },
      {
        "scopes": [  "device.read", "device.write" ],
        "resource_set_id": "987654"
      }
    ]
  }
}

And given the following placeholder:

{{ jwt:authorization/permissions/scopes }}

It would be expected that this resolves to:

issuer:device.read
issuer:device.write

Due to the nature of "flattening" arrays.
This however does not work, it resolves to no subjects at all..

The JWT placeholder implementation currently seems to only "flatten" arrays at top level.

It should work "recursively", in arbitrary nested JsonObject / JsonArray constellations.