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.
* Create the `codegen-core` module
* Move the `Version` class into `codegen-core`
* Rename `codegen` to `codegen-client`
* Rename `codegen-test` to `codegen-client-test`
* Move shared test models to common location
* Fix Smithy validation errors in `misc.smithy`
John DiSanti