Next steps for configuration

[dmlloyd]

The next step for configuration is to make the config groups with @ConfigRoot annotations be authoritative roots for configuration. This entails a few steps:

• Update ExtensionAnnotationProcessor to list config root classes similarly to how build step classes are currently done
• Create a build step to consume this file and produce build items
• Create a build step to consume the config root class build items and register them into the configuration tree (taking over this job from ExtensionStepLoader and possibly partially from ConfigSetup)

[dmlloyd]

Configuration root classes should be allowed to end in Config, which would be stripped off automatically (unless it is the whole name). This is because the class name is likely to conflict with the thing it is configuring otherwise.

Maybe we should allow a suffix for fields too, that gets automatically stripped, since e.g. class would be a pretty common key.

[emmanuelbernard]

Is that a first public release for some of these steps? If so let's label it.