Implement GET funding sources linked to a group

[thekaveman]

Implement GET endpoint for funding sources currently linked to a group.

The group ID is a URL path parameter.

Returns a JSON body response like:

{
  "list": [
    {
      "id": "9002b586-9c53-43c6-b274-5615adb856ee",
      "participant_id": "test_participant",
      "concession_expiry": "2024-07-03T00:00:00Z",
      "concession_created_at": "2024-07-03T00:00:00Z",
      "concession_updated_at": "2024-07-03T00:00:00Z"
    },
    {
      "id": "6003a697-1d64-54d7-a385-4726bec767ff",
      "participant_id": "test_participant",
      "concession_expiry": null,
      "concession_created_at": null,
      "concession_updated_at": null
    }
  ],
  "total_count": 2
}

Note the date fields concession_* in the list portion could be null if/when the linked funding source has no expiry information associated with its linkage.