Allow plugins to contribute Forge components

It would be nice if Forge components could be contributed through an extension point. This would allow components supplied by other projects (e.g. SwitchYard) to bundle their plugins in an Eclipse plugin and have them available automatically.

This would also help ensure the Forge plugins are compatible with the Forge installation used within Eclipse.

(I think the current integration, adding <user>/.forge/plugins, could leak incompatible Forge plugins into the Eclipse integration. Just my two cents.)