Consider defining structure of bound actions/functions

Bound functions and actions appear in json payloads as properties that start with "#" and may contain target, url, etc. These are not advertised as properties in JSON Schema, because such bound actions/functions can be "externally defined". This is okay because we specify additonalproperties=true, but we lack validation that the contents of a bound function or action is valid. Also, we prevent a service that knows it will never return additional properties from publishing that fact.

We could use patternproperties to enforce that the value of a property that starts with "#" is a valid payload for a function or action.

There is concern that adding this pattern property everywhere a bound function/action could appear might be heavy weight.

I would like to at least define the shape for an action/function, so that services that want to advertise actions/functions (either through pattern properties or by explicitly advertising actions/functions) can do so.