Consider using synchronous member of action affordances in Thing Descriptions

[benfrancis]

The WoT Thing Description specification has a synchronous member for action affordances which provides a Consumer with a hint as to whether the action behaves synchronously (output in the response to the invokeaction request) or asynchronously (a new resource is created to track the status of the action).

The WoT Profile specification also uses this member to distinguish between actions which provide a synchronous or asynchronous response.

Currently WebThings Gateway does not provide this member for actions, but it could provide it and set it to true since actions exposed by WebThings Gateway are always asynchronous.

This is not essential for W3C WoT compliance, but could be a useful hint for Consumers.