Generator composition needs to be redone and simplified

[karlvr]

There's a couple of different approaches to compositing the functionality of the generators and it's really confusing to work with. Particularly the type of the options method that extracts options from the CodegenConfig is hard to manage.

There must be a better way... I feel like I nearly had it nicely but it ended up being really complex.

e.g. the difference between how java-jaxrs-common uses the base generator etc and the way java-jaxrs-server uses java-jaxrs-common.

Maybe we can then extract the Jackson annotations out of java-jaxrs-common and they become a mixin module that provides template hooks.

[karlvr]

Stage one involved simplifying the types around the generators:

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