VM::Controller: add hook to render metadata in the same context

[chrisandreae]

This would permit metadata serialization to contribute referenced viewmodels.

[chrisandreae]

Oh, one awkward thing here is that this wouldn't place nicely with the prerender_json_view used to render cached content, because in that the references are already preserialized as json and there's no serialize context to contribute more to. That alone is probably a blocker for this idea, unless you've got any insight.

I suppose it could construct an ad-hoc serialize context and then carefully merge that with the preserialized references, but that's mildly painful.