smithy-rs

This is a pared-down version of #2809.

This is the code-generated version of the
david-perez/smithy-rs-service-config#2 POC.

This introduces a code-generated `PokemonServiceConfig` object which is
provided to the service builder constructor via
`PokemonService::builder(config: PokemonServiceConfig)`. This will
displace the
current `builder_without_plugins` and `builder_with_plugins`,
deprecating them.
We will eventually remove them.

The motivation is to have a single place where to register plugins and
layers.
Smithy traits can leverage this object to inject methods that can apply
plugins
and layers. For example, an `@authorization` trait implementer could
vend a
smithy-rs decorator that pre-applied an authorization plugin inside the
`${serviceName}Config` object, possibly exposing a method to pass in any
runtime arguments needed to configure the middleware.

## Checklist
- [x] I have updated `CHANGELOG.next.toml` if I made changes to the
smithy-rs codegen or runtime crates

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