Upsert operation

[fgalan]

An upsert variant of the entity creation operation should be included using ?options=upsert. For instance:

POST /v2/entities?options=upsert
{
  "id": "E",
  "type": "T",
  "A": {
    "value": 2,
    "type": "Number"
  }
}

will create the entity or update it if already exists.

Note that currently the same can be done using:

POST /v2/op/update
{
  "actionType": "append",
  "entities": [
    {
      "id": "E",
      "type": "T",
      "A": {
        "value": 2,
        "type": "Number"
      }
    }
  ]
}

However it involves more syntactical complexity (i.e. actionType and entities elements) as that operation is designed to a bulk update on entity (and a bulk of 1 is a bit missuse of the operation)

[fgalan]

Implemented by PR #3217