Introduce a "unit test" extension

[dmlloyd]

We need to introduce a fake extension for unit/regression testing the extension mechanism.

• Introduce the basic extension
• Exercise all the different config phases
• Exercise as many config types as possible including lists, maps, & primitives
• Use bytecode recording in both stages
• Be compiled & tested with Javac and then again with ECJ
• Be tested with native-image
• Include resources in native-image
• Create CDI beans and interact with BeanContainer
• Use substrate substitutions

(add more requirements here)

[starksm64]

I can make a start on this. Should this be located in extensions/test?

[starksm64]

core/test-extension was what was suggested on the call today.

[stuartwdouglas]

Is this still relevant?

[dmlloyd]

This is done.