Add skeleton for supporting operation level config (#2589)

## Motivation and Context
This PR adds skeleton code for supporting operation level config behind
`enableNewSmithyRuntime`.

## Description
The approach is described in
https://github.com/awslabs/smithy-rs/pull/2527

, where fluent builders
now have a new method, `config_override`, that takes a service config
builder. The builder implements the `RuntimePlugin` trait and once
`TODO(RuntimePlugins)` is implemented, allows itself to put field values
into a config bag within `send_v2`.

## Testing
Added a line of code to an ignored test in `sra_test`, which, however,
does check compilation.

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Co-authored-by: Yuki Saito <awsaito@amazon.com>
Co-authored-by: John DiSanti <jdisanti@amazon.com>