Reduce usage of CodegenTarget

[david-perez]

We introduced CodegenTarget in generators to branch off depending on whether we were generating a client or a server. Now that we're slowly unburying ourselves from the technical debt incurred by having core generators used by both projects in the codegen (now codegen-client) module, we can reduce its usage.

In general, the code is more maintainable by leveraging composition/customization injections/inheritance (probably in that order) in order to tweak a code generator in particular cases. This issue tracks the lofty goal of refactoring to these mechanisms instead of introducing if (target == ...), which hinders separation of concerns.

An ambitious goal would be for CodegenTarget to get entirely removed, but we'll see if that's attainable or desirable.