Access timestamp from custom LwM2M decoder microservice

When a decoder microservice is used as a custom action to process a LwM2M payload, it would be useful to have access to the timestamp if it it present in the SenML-JSON/CBOR message.

That is, if the payload is received by the LwM2M agent as the result of a send, notification, or read operation, and contains a base timestamp (bt):

[
	{
		"bn": "/72/",
		"bt": 25462634,
		"n": "1/2",
		"v": 22.4
	},
	{
		"n": "1/2",
		"v": 22.9
	},
	{
		"n": "1/2",
		"v": 24.1
	}
]

or a base timestamp (bt) and an offset (t):

[
	{
		"bn": "/72/",
		"bt": 25462634,
		"n": "1/2",
		"v": 22.4,
		"t": -5
	},
	{
		"n": "1/2",
		"v": 22.9,
		"t": -30
	},
	{
		"n": "1/2",
		"v": 24.1,
		"t": -50
	}
]

then the timestamp (i.e. bt + t) should be sent to the decoder microservice along with the resource value, e.g.:

{
   "args":{
      "resourcePath": "<<LWM2M Object Resource Path on the device>>"
   },
   "timestamp": "<<timestamp from resource payload>>",
   "sourceDeviceId":"<<device Id>>",
   "serviceKey":"<<The service key of the decoder microservice>>",
   "value":"<<The value to be decoded (hex string)",
   "status":"IN_PROGRESS"
}

In this way, the decoder microservice can set the time of any measurements, events, or alarms that it returns to the LwM2M agent.