Align with top-level 'link' field of the TD (for Prague WD)

[danielpeintner]

TD plans to add a new field "link" for Prague (see w3c/wot-thing-description#94)

We might consider adding this to scripting also.

[mkovatsc]

So far link is only planned for informative metadata than does not need processing by the runtime. For now, they all shall be simply passed on to the application.

[zolkis]

The API could look like this:

dictionary TDLink {
  required USVString href;
  DOMString rel;
  USVString mediaType;
};

partial interface ConsumedThing {
  sequence<TDLink> getLinks();  // throws on error
};

The TD is already fetched, that is why the method is synchronous.

[zolkis]

I have prepared a PR to add this. Let me know if I should file it.

[mkovatsc]

rel and mediaType should be optional, as I guess we also need this definition for ExposedThing.

RFC 8288 now leaves it open which target attributes exist; they may depend on the rel type or the link serialization (such as our TD). In the next step, we will probably need to allow for arbitrary attributes, as domain-specific contexts may define such attributes for the TD link serialization.

[zolkis]

By default all dictionary members are optional, so we need to make href a required property.

[zolkis]

Handled by #113.