Allow for more TM fields to be optional

[JKRhb]

In the context of w3c/wot-binding-templates#258, one result of the discussion was that there might be cases where information within a Thing Model is primarily aimed at implementors of Things, indicating that they should support certain features or extensions of a protocol, like CoAP's Echo option. In this particular case, a consumer does not benefit that much from receiving this information in a TD, while implementors could use this information as a set of constraints for the Thing's behavior. However, at the moment it is required to take over all definitions (expect affordances labelled optional via tm:optional) into the resulting TD, which is why having vocabulary that is only used by Thing implementors is not really possible at the moment.

I was therefore wondering if we could either expand the scope of tm:optional to other levels of a TM or whether it could be possible to label vocabulary terms as optional during instantiation in general (e.g., in a Binding Template document where a vocabulary term is introduced). I guess the former approach would be a bit more flexible and would also cause less burden for specification authors, although it could make TMs a bit more verbose in certain scenarios.

When it comes to the example use case mentioned above, this could lead to the definition of a form like the following, where we use a JSON pointer to indicate that the cov:usesEcho member is optional via a tm:optional at the form level:

{
    "@context": [
      "https://www.w3.org/2022/wot/td/v1.1",
      {
        "cov": "http://www.example.org/coap-binding#"
      }
    ],
    "title": "ExampleThing",
    "securityDefinitions": {
        "nosec_sc": {"scheme": "nosec"}
    },
    "security": "nosec_sc",
    "properties": {
        "example": {
            "forms": [
              {
                "href": "coap://example.com/example",
                "cov:usesEcho": true, // Note: Only a placeholder
                "tm:optional": [
                  "/properties/status/forms/0/cov:usesEcho"
                ]
              }
           ]
        }
    },
}

Alternatively, we could also consider supporting JSON Path expressions, using the respective JSON object of the form as the root node.
This would make the tm:optional array entry a bit more concise:

{
    "@context": [
      "https://www.w3.org/2022/wot/td/v1.1",
      {
        "cov": "http://www.example.org/coap-binding#"
      }
    ],
    "title": "ExampleThing",
    "securityDefinitions": {
        "nosec_sc": {"scheme": "nosec"}
    },
    "security": "nosec_sc",
    "properties": {
        "example": {
            "forms": [
              {
                "href": "coap://example.com/example",
                "cov:usesEcho": true, // Note: Only a placeholder
                "tm:optional": [
                  "$['cov:usesEcho']"
                ]
              }
           ]
        }
    },
}

Looking forward to your feedback on this – if this would make sense as a potential addition to the specification, then I would transform the content of this issue into a use case submission via the new template :)