Introduce a native image extension

[dmlloyd]

Description
Make native image be an extension rather than a dedicated Mojo. Make sure the extension can be explicitly disabled by a configuration property. Make sure all the current Mojo configuration flags are moved into the build-time configuration of the extension.

Implementation ideas
Produce the native image during the augment phase. Use a virtual build item as a pseudo target for the native image output. The virtual item would be declared as a final item if the extension is present and enabled. Build steps can use weak production to produce code that is conditional on native image being enabled.

[gsmet]

@dmlloyd hasn't this been addressed by Stuart's latest work on the subject?

[dmlloyd]

Yes, I think this can be considered solved.