Add support for type filter in push subscription

[neilj]

You can currently register what types you want to receive pushes for. However, this could still result in more pushes, and thus device wakens, then you need. This could be solved by allowing you to associate a filter object (the same as you would pass to /query) with the type in the push subscription. The server would then only send the push if the state of one of the objects changed matched the filter either before (destroyed, updated) or after (created, updated) the change.

The main downside of this is complexity of implementation; I think it would have to be optional and servers can still just always push whenever the type changes. That's OK, because it doesn't affect the client's implementation. It just means the client may get woken up slightly more often than necessary.

[neilj]

It was determined at IETF102 that this should be an extension.