Break up `RustCodegenDecorator` (#2099)

This change creates `ClientCodegenDecorator` and
`ServerCodegenDecorator` in `codegen-client` and `codegen-server`
respectively to replace `RustCodegenDecorator`. Client/server
equivalents are created to replace `CombinedCodegenDecorator` as well.

This eliminates the need for the `supportsCodegenContext` method since
the decorator interface is no longer generic, so the `ServiceLoader` is
now powerful enough to differentiate.

The largest benefit, however, is that now clients and servers can have
separate customizations.