Use the `SymbolProvider` and `Model` directly from the Smithy plugins (#1438)

Despite all the decorators and protocols that we have, there aren't any
`RustCodegenDecorator`s or `ProtocolGeneratorFactory`s that are
modifying the base symbol provider that the Smithy plugin provides. So
we can use directly the base symbol provider everywhere and remove the
methods from these interfaces to make the code a little bit simpler.

Likewise, there isn't a single `ProtocolGeneratorFactory` that
transforms the base model that the Smithy plugin provides.