#29814 Added mockgun mocker to Shotgun API

This adds a highly experimental 'Mockgun' Shotgun API mocker to the
Shotgun API. This is being moved across from the Toolkit unit test rig
into the Shotgun API to allow for a wider range of use and reuse.

Mockgun is highly experimental at this point. It only supports a subset
of the Shotgun API and we are likely to make big and potentally breaking
changes to this in the future.

Closes #87

Author: manneohrstrom