chore: enforce singular config object for resolver stack

[kuhe]

This PR represents a component of step (1) in the following process:

1. all config resolvers in Smithy and AWS SDKs will mutate a single config object (except the initial operation to generate the runtimeConfig)
2. AWS SDK enforces the change from (1) with integration tests
3. config resolvers and middleware functions are safe to refer to the config object, knowing it is a singleton and not a dangling reference to an intermediate object.