Add support for "expiry" of policy subjects

[thjaeckle]

In order to give access for a certain "authorized subject" only until a fixed timestamp, a Policy subject shall be optionally provided with an "expiry" timestamp (being an ISO-8601 string).

Once the "expiry" timestamp was reached, the subject shall be automatically removed from the policy (that change being persisted) and as a result this subject would immediately no longer have access.

Example Policy:

{
  "policyId": "my.namespace:policy-a",
  "entries": {
    "owner": {
      "subjects": {
        "nginx:house-owner-user": {
          "type": "This is the owner of the house"
        },
        "nginx:neighbor-user": {
          "type": "This is the neighbor which shall manage the smart house during a vacation",
          "expiry": "2020-11-26T11:00Z"
        }
      },
      "resources": {
        "thing:/": {
          "grant": [
            "READ",
            "WRITE"
          ],
          "revoke": []
        },
        "policy:/": {
          "grant": [
            "READ",
            "WRITE"
          ],
          "revoke": []
        },
        "message:/": {
          "grant": [
            "READ",
            "WRITE"
          ],
          "revoke": []
        }
      }
    }
  }
}