Create an AppD reference implementation

## Enhancement Request

It is advisable that any standard provide a reference implementation that may be used to confirm or compare the expected behavior of an implementation of the standard and/or be used as a base for other implementations. The AppD is one such standard and it is likely that the provision of a usable reference implementation will help drive its uptake throughout the financial services desktop software community.

In the past, a java implementation of the AppD was provided at: https://github.com/FDC3/appd-poc. This is not currently maintained, nor included in the FDC3 repository. Reviving this implementation, or providing a new one (ideally based on Node.js) is desirable.

### Use Case:
- Reference implementations may be used:
- directly, saving a firm from implementing their own AppD
- as a starting point by firm implementing their own AppD
- as an example of expected behavior from AppD.