Support for watch controls and coding watch functions

[neilcsmith-net]

A watch is a special type of function control to be used for accessing and visualizing data from a component. They should return a result map, with data in textual or binary form, along with metadata such as mimetype required to decode. They may accept an optional configuration map as input to allow callers to request specific data types or sizes.

Support for coding watch functions should allow for async response, either for background processing of data and/or waiting for data to be available (eg. passing through a port).

Depends on #68