Support local/relative tm:ref references in WoT TM support

[thjaeckle]

The WoT specification defines that when using "tm:ref", it is also possible to reference local properties/actions/events, e.g.:

From the WoT spec example:

{
    "@context": "https://www.w3.org/2022/wot/td/v1.1", 
    "@type": "tm:ThingModel",
    "title": "Multi Sensor",
    "properties": {
        "genericTemperature": {
            "type": "number",
            "unit": "C"
        },
        "innerTemperature": {
            "tm:ref": "#/properties/genericTemperature",
            "title": "The inner temperature",
            "minimum": 10
        },
        "outerTemperature": {
            "tm:ref": "#/properties/genericTemperature",
            "title": "The outer temperature",
            "description": "The outer temperature is measured in Kelvin",
            "unit": "K"
        }
    },
    "tm:optional": [
        "/properties/genericTemperature"
    ]
}

Ditto currently only supports referencing external references, expecting that "tm:ref" always starts with an URI.

Add support for this to be more standard compliant.