Mechanism to facilitate dynamic URI resolution for Thing Model extension and import

The algorithm for the derivation of Thing Description instances from Thing Models, as outlined in the [td specification](https://www.w3.org/TR/wot-thing-description11/#thing-model-td-placeholder) does not allow for the resolution of {{placeholders}} that are part of the links section of a Thing Model.

Example:

In the current implementation (and according to the specification steps), such a trying to instance the Thing Model below, would result in an exception since affordances are fetched before placeholder replacements:

```
{
"@context": "https://www.w3.org/2022/wot/td/v1.1",
"@type": "tm:ThingModel",
"title": "simple_example",
"description": "Example thing with a single property",
"version": {
"model": "0.0.1"
},

"links" : [{
"rel": "tm:extends",
"href": "http://{{MODEL_REGISTRY_ADDR}}/some_other_model.tm.jsonld",
"type": "application/tm+json"
}],

"properties": {
"random_value":{
"description": "a random value between 0 and 100",
"type": "number",
"minimum": 0,
"maximum": 100,
"observable": true,
"forms": [{
"href": "mqtt://{{MQTT_BROKER_ADDR}}",
"mqv:topic": "{{THING_UUID_V4}}/properties/random_value",
"op": [
"observeproperty",
"unobserveproperty"
],
"mqv:qos": 0,
"mqv:retain": false,
"contentType": "text/plain"
}]
}
}
}
```

However, a dynamic replacement of the {{MODEL_REGISTRY_ADDR}} could be useful for the development process of WoT applications (my current use case) or when multiple registries exist that may be searched iteratively.

Could placeholder replacement be supported for this use case?
If not, could some other mechanism be added to the ThingModelHelpers to redirect affordance resolution attempts during the partialTD generation process?