Remove middleware code when generating the orchestrator exclusively (#2723)

## Motivation and Context
This PR removes all the middleware code from generated clients when the
`smithy.runtime.mode` flag is set to `orchestrator`. It also changes the
`aws-config` crate to use the fluent client instead of the Smithy client
for STS and SSO based credential providers.

The `polly` test no longer compiles in orchestrator mode with these
changes since presigning directly references middleware code. This will
get fixed in a later PR.

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