Eliminate dependency on codegen-client from codegen-server

[david-perez]

Now that codegen-core is a thing, we should work towards eliminating the dependency on codegen-client from codegen-server. So the goal is to drive this number down to zero:

find . -name "*.kt" | xargs grep -n "software.amazon.smithy.rust.codegen.client" | wc -l

These are candidates to be moved to codegen-core and be made more general if possible, reducing our usage on CodegenTarget too: #1765.