Consolidate license endpoints + DTOs

[overheadhunter]

Currently we have a collection of different endpoints and DTOs serving similar purposes:

• GET /billing (used in AdminSettings.vue):

  {
    "hubId": "hub3000",
    "email":, "admin@example.com",
    "licensedSeats": 42,
    "usedSeats": 23,
    "issuedAt": "2020-01-01T00:00:00Z",
    "expiresAt": "2030-01-01T00:00:00Z",
    "managedInstance": false
  }

• GET /license/user-info (used in VaultList.vue and VaultDetails.vue):

  {
    "licensedSeats": 42,
    "usedSeats": 23,
    "expiresAt": "2030-01-01T00:00:00Z",
  }

• GET /config/ (used in whole app):

  {
    "keycloakUrl": "https://example.com/kc",
    "...":, "more stuff",
    "entitlements": {
      "seats": 42,
      "...": "more entitlements"
    }
  }