Design and implement the next-generation Forge scaffold plugin

This issue is to track the design and implementation of the Forge next-generation scaffold plugin for Forge 1.x. Some of the proposed features here may not be implemented in Forge 1.x through constraints introduced by 1.x, and hence would be dropped and implemented in Forge 2.x; see FORGE-808 for the 2.x addon.

Some of the proposed features include:

• The ability to use Freemarker templates to generate the scaffolding.
• The ability for users to modify the said templates to extend the existing templates provided out-of-the-box by the scaffold provider.
• The ability to scaffold from any resource, and not just JavaClasses.
• The ability to compose a scaffold provider from existing scaffold providers. (This particular feature may be omitted in this 1.x plugin).