Allow custom LwM2M decoder microservice to create/update arbitrary inventory fragments

A custom decoder microservice can return dataFragments to the LwM2M agent in order to update the managed object, but this is only capable of updating the fragment specific to the LwM2M resource, e.g.

"dataFragments":[
{
"value":"Hello World",
"key":"/45678/0/1234"
}
]

will update the fragment:

"c8y_Lwm2m_45678!0": {

"1234": "Hello World"

}

It would be useful to be able to create or update the value of properties in any arbitrary fragment in the managed object, e.g.

"myCustomFragment": {

"myCustomProperty": "Hello World"

}